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AndrewHighlands
Member
# Posted: 6 Nov 2020 05:37
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I'm setting up an off-grid electrical system and I am relatively weak on electrics know how.
I am intending to upgrade an existing caravan system which runs on a single 110Ah lead acid leisure battery (for simplicity). I'm looking to at least double the capacity by having a second identical battery in parallel.
The initial battery is not new and I wondered whether in this case the system would work okay with one battery that was at optimum performance and another that was slightly 'old'. I don't want to buy two brand new batteries if the old one still has some life in it. But then, I don't want to damage the new battery.
So can someone tell me please, would it be detrimental to the new battery if I linked it up in parallel with the old battery? Would teh system still work fine?
Thanks,
Andrew.
PS, is there a forum search function on this website? I can't find one, and I suspect there are other questions about battery care where I could have had my question answered.

Nobadays
Member
# Posted: 6 Nov 2020 08:23
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Never a good idea to mix old and new batteries, or batteries of different capacities such as a 110ah with a 150ah. (Or batteries of different brands but that is another discussion. )

As batteries are used/cycled they lose capacity, your 110ah battery really isn't capable of that whole 110ah anymore. So even putting an identical battery in parallel you will still have to unevenly matched batteries. When charging, each will require different degrees of charge. In the end you will likely either fry the old battery and or shorten the life of the new one as it may never reach full charge... always under charged kills batteries. If you want to proceed, you could get a couple of A/B battery switches to isolate charge/discharge of each battery. Use off one, one day while charging the other and visa versa.

Another thing to think about is your ability to charge double the ah of batteries. Even if your system is well designed to charge a 110ah battery, can it actually charge 220ah efficiently.

Nobadays
Member
# Posted: 6 Nov 2020 08:25
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Oh... search feature is a link in the top bar... forums - profile - reply - search .....

paulz
Member
# Posted: 6 Nov 2020 10:17 - Edited by: paulz
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Second what Nobadays said. Put the new battery on a switch, great idea. If you used it enough, it would get to the same condition as the old battery, which a hydrometer might tell you. But that's just a guess.

gcrank1
Member
# Posted: 6 Nov 2020 21:19 - Edited by: gcrank1
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Simple system, no worries, we all started somewhere, and made our mistakes in the learning curve. You will too.
It has been said that one will kill their first battery bank.....pretty much true, ime (btw, doesnt matter how many batteries you have, it/they are the 'bank').
The RV stuff was good basics for me, have you been using such or just have it available so want to start there?
Fwiw, my 4 x 130ish ah agm ups bank is in need of replacement so I have been limping along since fall on
the last 2, then 1 (getting the really bad ones out of dragging the whole bank down; ie the strong battery(s) are diminished by the weak battery(s)) and now it is in the 8x12 toolshed/workshop for pretty much just lights (literally 'light use', lol).
The cabin is now on a well used 100ah marine dual purpose donated by a fisherman friend, he knew it wouldnt be any good for trolling come spring and I might could get some amp hours out of it. As said, it doesnt have the 100ah that it once did....the led lights are fine on it, the ceiling fan spooled up a little slow.
Since it is already compromised I paralleled in another donated used diesel truck big battery and did things ever improve. Btw, I didnt know that some new trucks have oem onboard inverters! So this one is also a dual purpose battery. I suspect the 'cold cranking amps' end of this type battery helps the inverter on the start up of stuff. No, they dont match At All. But ya know, it works better than the 10yr old big agm did! I suppose one will be 'hurt' by the other one but meanwhile I have amp hours to use, on the cheap, and, here's the key, this is providing me with my admittedly simple and low-end power for a few days (well, mostly evenings now, my electric use is really light during daytime).
Your battery is already old and on its way out, put another in the loop and what do you have to gain/lose? You will get more amp hours to use, if that is what you really need. But do you? My pref is to only draw my bank down by 25-30% per day, NEVER go past 50% and start recharge asap the next day, even if I only used 10%, to bulk quickly, if possible and let the rest of the day 'top up'. If that means I run my 2K watt inverter gen with a smart 2/10/20 amp charger on 20 for an hour in the morning, so be it. I will use that gen running time for tools, microwave, toaster, coffee pot, etc (one at a time, mind you). The charger is on the gen line to cabin so whenever the gen runs the charger is running, no downside to that!
I can also rotate the batteries, as as was mentioned; ie, bring one home to really charge before the next trip up to the cabin. We did that regularly on our 1983-84 off grid build. I had no i-net for info/research, no solar and basically no clue other than some little trolling motor on a 'marine battery' experience. I killed a few batteries....I run em until the old rv incandescent edison base bulbs were too dim to use then switch to the kero lamps.
So, conserve elec first, dont try to use energy hogs on batteries, Keep It Simple.
Dont ever parallel more than 2 batteries together.
Dont oversize an inverter if you intend to turn 12vdc into 110vac and dont even bother with a cheap 'modified sine wave' inverter, get a Pure Sine Wave. Some people think they can run a 1000 watt, or more, inverter on a 100ah battery; well, ya can, for a very short time. Dont oversize an inverter for the bat-bank you have.
There's more....much more.
Simple is good.
More complicated can still be good, but....its gonna cost ya in money and time/work. That is the downside to being your own power company.

AndrewHighlands
Member
# Posted: 9 Nov 2020 04:09
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Thanks very much for all that useful information.
I've been trying to get my head around upgrading this system for months and decided a while ago to start simple and learn from practical experience (because with 'theory' I've been making little progress).
The current caravan battery might be kaput or at best not in a very good state. I was last on-site about two month ago and it was left at about 60% charge.
I bought a small pure sine wave generator but I have not used it to charge the battery yet. My plan is to buy ca. 300W of solar panels then top up using the generator when necessary.
The point about capacity to re-charge using renewables is something that I had not thought about. Having a switch to charge batteries separately is something that I had not thought about. Thank you. I assumed that the batteries would just trickle charge.
One question:
Would there be any benefit in having the 12V system of 220Ah comprised of two 6V batteries in series, rather than two 12V batteries in parallel? I've seen some good deep cycle 6V batteries that I an inclined to buy it I have knackered the existing 12V battery.
Thanks,
Andrew

Nobadays
Member
# Posted: 9 Nov 2020 08:13
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Andrew... the biggest advantage to using 2/220ah 6v batteries in series over 2/220ah 12v batteries in parallel is longevity. Most 6v deep cycle batteries, especially those intended for solar applications, are more robust in design than the typical 12v marine deep cycle battery. The 6v plates are generally considerably thicker. Batteries fail because as the charge/discharge material is shed from the plates, thicker translates to longer life if both the 6v and 12v are treated equally well.

In your scenario, that is the main advantage.

gcrank1
Member
# Posted: 9 Nov 2020 16:53
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Yes, by all means go with the 6v, 2 in series to make one big 12v bank! If you want even more amp hours capacity go with two banks like that paralleled together.
As to charging your current battery off the inverter/gen, use the gen to run a 110v (if it is a 110v unit) smart charger, dont use any 12vdc hook up on the gen, it will take forever......
Pls reconsider your charge methodology; ie. bulking on solar and 'topping up' on gen. The solar is really a 'trickle charger' already. do it the other way, bulk up on the gen and let the solar do the absorption to float charge. To run a gen as long as it takes to do the float can take long hours.
What I try to do is get a full charge into my bank by day's end so I have whatever it's capacity is for the night. I try to not deplete it by more than 25-30% at night, that means a quicker recharge the next day and longer battery life. If you get in the habit of depleting to 50% you are going to need more panels, a big smart charger and a big bill for replacing batteries prematurely (unless to go LifePo, or whatever those terribly expensive new generation batteries are called). Btw, as I understand them they dont do well in cold and need special chargers. So, if they are in your home where the temp is moderate, good to go, if in an occasional use place that regularly gets pretty cold, not good; they will have to be warmed up before you can start to recharge them.
A useful rule of thumb with tradition wet cell batteries or agms is the C-10 discharge/recharge thing. C = charge, the 10 (in this case) is 10% of the battery bank ah capacity. For healthy battery use you can discharge up to 10% for x amount of time to the depletion state of charge (for me, that 25-30%) and you can recharge at that 10% and not hurt the battery.
So, your 220ah bank at C-10 would be a max discharge of 22amps and a recharge the same (give or take a bit is ok). That is some pretty healthy use; remember that is max and you will go lots longer on less amps draw.

Nobadays
Member
# Posted: 9 Nov 2020 17:39
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I second the use of a smart charger running off your generator. As Gcrank says, the generator 12v chargers are a notoriously poor option to charge a battery bank.

A correctly sized PV system in combination with a good quality MPPT solar charge controller (SCC) is more than a "trickle charger." If done right can bring your bank up to float in a few hours.

We use "those terribly expensive new generation batteries" (not expensive if you factor in how long they last) and with 2250 watts of PV running through a 60watt SCC we can bring our bank back to full charge in about ~4 hours.... we commonly discharge roughly 20% between charging periods of sun. But, as in today with no charging to speak of because of clouds and snow the last 24 hours, we can take 80% out of our "new fangled" lithium batteries and do no damage to them at all.

We keep ours under the staircase where they remain warm, (they don't off-gas and don't need regular maintence.) If we leave the cabin for several days in the winter, we do indeed need to wait until they are above freezing to commence charging, that is usually just overnight. But, they can be left at 80% capacity for months, cold or warm and they lose virtually no power at all. And they can stand very rapid charge and discharge without effecting their performance.

Sorry Gcrank.... you pushed a button! Many of us on here are lithium battery users (mine are upcycled Chevy Volt modules... no landfill but back in use) and we really, really like them!

ICC
Member
# Posted: 9 Nov 2020 18:06
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Quoting: gcrank1
reconsider your charge methodology; ie. bulking on solar and 'topping up' on gen. The solar is really a 'trickle charger' already. do it the other way, bulk up on the gen and let the solar do the absorption to float charge. To run a gen as long as it takes to do the float can take long hours.



IF the solar consistently ca NOT recharge the days use with normal sunshine then IMO, the solar capacity is insufficient. A properly sized solar and storage system does not even need a generator. I have generators because I have other needs for a generator. The home and workshop run off solar. When I had lead acid batteries I even did the ocassional equalizations from the solar.

I have friends who do not even have a generator. 100% solar or in one case about 33% from wind.

Now, IF one does need to run a generator to recharge lead-acid batteries, it is more fuel efficient to run the generator for the bulk stage charge and then let the solar finish off. If one has lithium batteries a generator can be useful too, but there is much less concern as, (a) lithium can be discharged more deeply and (b) when the sun comes back lithium charge faster from the solar than lead-acid do.

While it is quite true that lithium should not be charged at too low a temperature that is not a deal breaker. Lithium, especially LiFePO4, can be stored in an indoor, warm space and do not need ventilation like lead-acid. Li batteries may cost more upfront but they can provide a much longer service life with no hassles of keeping the fluid level up.

mj1angier
Member
# Posted: 9 Nov 2020 19:07
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If you have not purchased your inverter yet, look at inverter/chargers. Your generator feeds it and the it feeds the cabin and charges battery at same time. When you turn off genny, it uses the battery to power cabin. Run solar to battery like normal.

Most of the time our 24v lithium battery will last a weekend of use for us. In summer we run the generator in the late afternoon/ evening to power ac ( I can't sleep if it's too hot) and it feeds excess power to battery.

gcrank1
Member
# Posted: 9 Nov 2020 19:11
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Oh, I would love to have those LFePO4 batteries! Not dissing them at all.....but I cannot justify the cost for our modest uses and on a fixed income big up front expenses rule out many things.
Yes, I agree too that a 'correctly sized', and sizable system, is more than a trickle charger but for what the Andrew has said are his parameters and equipment now I think he is much like mine and in an effectively trickle charge zone. I can say Ive seen 11amps charge from 2 x 102watt 12v panels in series for 24vdc into an mppt controller (converted there to 12vdc) when the bank was depleted and the sun was optimum, well past 'trickle', but that doesnt last all day long so when I look at the amps gain in a good day the average is more of a trickle. and on a bad solar day it is.....dismal.
That 11amps seemed pretty good to me, like my 'inherited' 10 yr old and now rewired and abbreviated system is working decently (I have 6 of those panels with a potential of some 36amps). In effect I think I have this down to a pretty basic and functional system, geared for some 'growth' and my big obstacle seems to be an adequate battery bank to utilize what the panels and controller are capable of. And that is gonna cost me, fla, agm or lith on an ascending scale. If I had to buy all this stuff today I think Id be going in light like Andrew, the learning curve can be pricey. Correctly sized panels and controllers might be a bargain now compared to 10yrs ago but batteries are the Big Expense so overall buy-in is anything but cheap.

Brettny
Member
# Posted: 10 Nov 2020 12:29
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For charging off a generator I have a 1kw sportsman inverter and a 30a RV onboard charger. I only bulk charge at 30a with the generator and get at least 8hrs run time on 0.55gal of fuel. More pannels would be nice but so would more sun. I have 470w of solar on our small roof. I cant fit any more and even then they see sun for 3hrs.

AndrewHighlands
Member
# Posted: 16 Nov 2020 04:46
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Thank you all so much!
I have two questions related to this helpful advice:

1. I have been now looking at 'smart chargers'. I can't see what they do and what the benefit. There are lots of brands, some which have Bluetooth and deliver charging regimes. I don't want any of that, and I should make it clear that I have always been one for keeping it simple so that there is less that can go wrong. This is particularly so with gadgets having complicated PCBs. Others have said here that to charge from the generator will take a long time.
Though I have not yet used my generator, it is a sine wave 230VAC with a charging point also for 12VDC. The manual gives a table which states charging times for 12V batteries based on hydrometer measurements of specific gravity. This lists times of around one hour (plus or minus a bit). Though it is a bit fiddly to check electrolyte, I can do this, and the charging time seems reasonable - much better than the other way of plugging a basic trickle charger into the 230v socket and connecting this to the battery terminals. So, what is the benefit of the smart charger and what do they do which the direct to 12V generator socket does not? I'm looking really for something that I can manually plug in for a while to keep the battery in good shape.
PS, I once read that fast charging is bad for the battery; though my head was sort of swimming with battery theory at the time, so I can't remember why. And, perhaps more pertinent, some of these smart chargers cost more than a new battery.

2. Forgive my ignorance, but if I buy two 6V batteries and connect in series, do I charge the bank as a 12V unit, or do I charge each 6V battery individually? If the latter, will my generator outing at 12V work on a 6V battery, and similarly would a smart charger have to be specific to 6V?

Thanks again. What a nice community this!

ICC
Member
# Posted: 16 Nov 2020 09:09
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Quoting: AndrewHighlands
2. Forgive my ignorance, but if I buy two 6V batteries and connect in series, do I charge the bank as a 12V unit, or do I charge each 6V battery individually? If the latter, will my generator outing at 12V work on a 6V battery, and similarly would a smart charger have to be specific to 6V?


Two 6 volt batteries connected in series are used, or discharged in series at 12 volts AND charged connected in series at 12 volts.

Nobadays
Member
# Posted: 16 Nov 2020 09:46
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Quoting: AndrewHighlands
Though I have not yet used my generator, it is a sine wave 230VAC with a charging point also for 12VDC. The manual gives a table which states charging times for 12V batteries based on hydrometer measurements of specific gravity. This lists times of around one hour (plus or minus a bit).


Don't know anything about your particular generator but a charge time of around one hour is incredible.... of course with this we don't know if the battery was 10% depleted or 50% depleted. Is it possible your generator has a smart charger built in?

A smart charger - over simplifying - charges your battery at the best possible input levels so it charges quickly and safely. There are generally several "steps" or amperage levels of input from high amperage fast charging to a trickle charge. All this is done while the charger monitors the battery insuring, again a safe and quick charge that best preserves your battery health.

What ICC said on question 2.

gcrank1
Member
# Posted: 16 Nov 2020 11:04 - Edited by: gcrank1
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Perhaps somewhere in your inv/gen specs it tells what amps output the 12v tap is, that would be the max amps it can output. Ime those are relatively low outputs and not nicely 'regulated', just convenient for some light duty charge up if your car battery runs down and you dont have someone close for a jumpstart; ie, let it run for a while to get a surface charge good enough to start up.
Battery: no matter how many hooked together it is your 'bank' and the voltage is the sum of how they are hooked together (after all, a 12v battery is just 6 cells internally joined). You treat the bank as one big battery.
For deep cycle use use the C-10 rate, that is:
Total amp hours of the bank divided by 10; that 'rate' is the max amps you should discharge at and recharge at.
Example: 2 x 6v in series = 225ah/10= 22.5amps.
Note that it takes more amps and time to recharge than what you take out, so you use a factor of x 1.1
That is, if you pulled 20 amps out over 2 hrs it will take you more than 20 amps recharge rate. If your 12v tap on the inv/gen only puts out 6 amps it will take maybe 4 hrs to put it back in I dont want to run a gen that long for 20 amps.
And as a battery comes up to full charge it slows on how much charge it can take so the time to top it off gets lonnggg. That last 5-10% can be glacial.....
The early stage charging, the bulk, up to around 85% can go pretty quick with enough charger. Using my C-10 example above you could charge at 22.5 amps (it isnt a 'hard' rule, there is some give on the rate) so with a 20-25 amp charger it would bulk it back in a little more than the 2 hours you took it out at. Its pretty much mathematical. And you want to recharge a wet cell battery asap after discharge, sitting in a partial state of charge kills those batteries.
After the 'bulk' charge the smart charger steadies out for an absorption/acceptance stage then kicks back into a float/trickle stage to finish the top off and hold for maintenance. This maximizes the charge effect to the battery and saves its life (and your money).
So you see that your charger max rate needs to sorta match what your battery can take. Each manufacturer has their own parameters, with some commonality. A C-10 rate may not be the actual max your particular battery can take, or the next one you buy. The nice thing about C-10 is it wont get you into trouble!
My smart charger came off Amazon, it has 2/10/20 amps selection for 12v fla. agm and gel batteries with all the high tech algorithms. It is a kinda fancy automotive charger. I bulk my battery up the next morning and whenever the inv/gen runs, like when running tools, and then let the solar panels work all day doing the 'acceptance to float' thing. If my use was modest (I take a dig-volt read on the battery each morning and sealed batteries you cant hydrometer read) I may just let the solar do its thing, no gen time required.
Works for me, ymmv.

AndrewHighlands
Member
# Posted: 17 Nov 2020 19:38
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Sorry to have to ask this, but would 'gcrank1' or 'Nobadays', mind sending me a hyperlink to the type of smart charger you mean, currently for sale on Amazon (of somewhere else on the web)? I have been searching, but I'm getting all kinds of different gadgets, a lot of which are Bluetooth enabled. I don't own a smartphone, so don't want anything like that.
Thanks,
Andrew

Nobadays
Member
# Posted: 17 Nov 2020 20:52
Reply 


Here is a link to a Go Power 4 stage 35amp. They make larger chargers but a 35amp would likely be great for your small battery bank.

HERE IS THE LINK

gcrank1
Member
# Posted: 17 Nov 2020 20:52
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Hyperlink?? Im just an old guy half-past luddite.....
Look for the:
Clore Automotive PL2320
About $82 US

AndrewHighlands
Member
# Posted: 18 Nov 2020 02:53
Reply 


Thank you both.
I looked at my generator manual again. I was wrong. The one hour (approximately), relates to near maximum charge state, and the table only gives three sizes of battery, the maximum being 47Ah. So, a smart charger it is!

Brettny
Member
# Posted: 18 Nov 2020 09:05
Reply 


I dont see any benefit of a smart charger in this application. If I'm burning gas and putting hours on a generator I want a charger to output max amps. Anything else and il just let the solar do it. I wouldnt even try to get the batteries upto 100% only get what they need to last until either the sun comes up or the next generator run cycle.

My Iota 30a RV charger has a plug that allows 3 stage charging or max 30a charging. I always use the max charging. I'm not sure if this is considered a smart charger but I dont use that feature anyway.

mj1angier
Member
# Posted: 18 Nov 2020 09:18
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I think that any charger you get needs to be able to do lithium batteries. If down the road prices drop to the point you want to invest in them, it would be nice to have a charger that can safely charge them. But that is just me

gcrank1
Member
# Posted: 18 Nov 2020 09:24
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For me it was I have several old school chargers, and the typical rv converter/charger that is often regarded as having very poor charging characteristics. If I was buying a new one it was going to be smarter than that stuff and multi-use, not just for the off-grid cabin when the gen runs; ie, I can use it at home on grid too and not have to always remember to check my bat soc when its running. I equate the smart charger to the solar charge controller in how it 'regulates' without me monitoring at all.
I agree that anyone running a gen to try to get a battery to 100% is not being 'gen-efficient'.

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