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Small Cabin Forum / Off-Grid Living / Lithium battery ?
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# Posted: 10 Aug 2021 17:18 - Edited by: mj1angier

So I have one 24v 100 ah battery that was put in service about 16 months ago. It does about 80% of what I need- some times we have to kick genny on on Sat afternoon to make it to Sunday if we need AC much.

Can I add a second battery like the first one to increse depth of power? I know it not good with lead acid batteries from new to old but not sure about Lithium. and if I can't same maker, will another 24 v 100ah be ok? Will the BMS make them play nice.

If not I think I will just bite the bullet and get 2 of these : sr_1_43?dchild=1&keywords=12V%2B300Ah%2BPLUS&qid=1628630281&sr=8-43&th=1

# Posted: 10 Aug 2021 17:41 - Edited by: gcrank1

LFP is not as sensitive to adding more bats like LA can be.
Not to your inquiry but a use question.
How about using the gen to run the heavy draw stuff; ie, the ac, fridge, etc. and have the bat run everything else? It should last a whole lot longer and maybe your gen use in hours would be similar?
Btw, before I had the solar finalized whenever the gen was running so was the smartcharger to the battery. We could go day after day like that when I didnt deplete the bats so deep.
For us the key to off grid cabin electricity is energy conservation. But having been from 1 LED lightbulb on up to now a microwave, toaster, and a 1k watt old coffee elec perc as a hot water tank as well as fans, usb chargers, radio and tv you can tell we are not deprived of the mod-cons. Sensible usage on the bats and the 1700/2000w inv/gen have made all the diff.

# Posted: 11 Aug 2021 07:23

As stated above, No Problem adding battery packs to FLP over time. Unlike Lead, each pack (Cells, BMS contained within) is an independent entity.

They should be close in capacity though. For example 100AH + a 200AH will result in issues when fully charged or discharged as the Lower AH pack with hit empty or full 1st and pending on condition the bigger battery pack would attempt to cross charge, losing overall capacity.

The Battery you Linked to:
WARNING ! Too good to be true IS Often too good to true !

Please LOOK at this Youtube Playlist (Starting from bottom for most recent) to see tests & Teardowns on PreBuilt Batteries. SOK, AmperTime and a few other "Value Priced" batteries rate Really Well while others are a disaster.

Many of the PreBuilts (most sadly) tell you to Hi Charge the LFP cells. This is VERY BAD !

Too many (most) do not understand that LFP "Voltage Limits" are 2.500 @ the bottom & 3.650 @ the Top. But that the Nominal Working Voltage is 3.200 with the Working Voltage Range of 3.000-3.400 (Grades A & B) or 2.900-3.500 for Grade A+.

All the people who insist on charging Battery Assemblies to 3.650Vpc and wonder why they Slam into the Runner Cell Wall and trip their BMS'. While commercial assemblies like BattleBorn are Grade A++ cells, the low end are not that likely. (Safe assumption I think).

DEF: A Runner Cell is a cell which will reach either 100%SOC Full State or 0% SOC Empty State before the other battery cells within a pack, resulting in a BMS Disconnect to protect the cells & assembly.

* Also a lot of these Ampertime & Other Pre-Built battery packs also like to push 14.6V (3.650Vpc) which is Max Limit and "technically" over voltage for the Working Range. If this fails & triggers a Runner (hard to see with dumb BMS) it's not good BUT it's also going to result in Less Cycles ! due to 100% Voltage Limit charging and we know that above 3.550 is pretty much pointless even with perfect 100% Grade A+++. 3.525Vpc / 14.1V is the top working range for A+ cells and max lifecycles, right.

I tinkered a lot and Thrash tested (semi abusive) various things including aggressive and mild charge profiles to push edges. I came up with a Charge Profile which is pretty solid, prevents runners (unless they are really bad but that's usually another issue too)

My Profile (with a MidNite Classic-200)
Divide Values X2 for 12V. Multiply X2 for 48V.
Absorb: 28.2 for 15 minutes (3.525vpc) (some call this boost)
Equalize: OFF
Float: 27.9V (3.4875vpc)
MIn Volts: 22.0 (2.750vpc)
Max Volts: 28.7 (3.5875vpc)
Rebulk Voltage: 27.7 (3.4625vpc)
End Amps: 14A (*1)

(*1): End Amps is calculated from the Highest AH Battery Pack in a Bank. IE: 200AH X 0.05 = 10A 280AH X 0.05 = 14A.
NB: Victron Forum discussion says EndAmps = TailCurrent
This get's the bank charged to full with high amps (Constant Current) and then float (Constant Voltage) tops off so the cells are on average between 3.475-3.500. I am running 7/24/365 so float is used up by the Inverter + provides whatever the packs will take to top off.

** Coulumbic Efficiency for LFP is 99%

LFP (LiFePO4) Voltage Chart

BTW: The DIY Option exists and you can really save a pile and make a high-quality battery pack yourself. It really is not that hard to do. Attached you will find a document which explains How-To and much more. Many of us use cells provided by this Very Well Known Good Vendor who actually sells Factory (Manufacturer) Grade A+ Matched & Batched cells with Factory test reports.

The Vendor is Amy Wan @ Luyuan Tech in Shenzen via Alibaba.

Example to Build a 12V/100AH Pack.
4x 105AH Grade A+ Matched Cells. $199 USD
1x JBD 12V/100A Smart BMS $52 USD (+ for Bluetooth or PC interface.)
Total from China: $251 USD + S&H S&H Note
1X 200A Fuse (MRBF $37 USD)
Two Battery Terminal Lugs ($20 USD)
Casing - Open Option

* S&H Note
You MUST use Chat or Messenger on Alibaba to get a Proper & Accurate S&H quotation. The Shipping Guesstimator is way out of whack.

BlueSea MRBF Fuses: $37 USD

Summit Racing SUM-CSUMG1431 - Summit Racing® Bulkhead Cable Connectors: $20 USD

Hope it helps, Good Luck.

BTW: For much more information on LFP and even building your own can be found at
Luyuan_Tech_Basic_Li.pdfAttached file: Luyuan Tech LifePO4 Assembly guide

# Posted: 11 Aug 2021 20:53

awesome write up Steve, appreciate your knowledge very much and your willingness to share even more.

# Posted: 11 Aug 2021 21:36

Thanks for all the info guys!
We do charge them when ever we run the genny but it's tough for my wife to start (bad back), just easier to just hit main power button. I think just adding the other battery will get her thru the weekends she goes by herself.

I am not going to try and build one- too many other projects already, lol.

# Posted: 11 Aug 2021 21:45

My wife has skills, but power system management is not one of them. Now give her a switch and she's good too!
Guess I have job security
Good on her that she goes alone and likes to be there.
Yeah, buy her another battery.

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