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Small Cabin Forum / Off-Grid Living / Charging up batteries with a generator
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# Posted: 4 Sep 2011 10:47 - Edited by: mattthehairy

Hello all, we currently have an off grid solar system in addition to our utility power. Eventually we will be totally off grid, but aren't quite there yet. We had a hurricane roll through our area and our land is like a war zone with down trees and power lines everywhere. Today is day 8 without power and thanks to our little off grid rig things haven't been too bad for us.

The problem is that we don't have quite enough PV panels to recharge our batteries all the way in a single day under our current elevated usage so we've been running a bit of an energy deficit. The first week was fine, but as we roll into week 2 our batteries need more love than we can provide.

Enter the generator. It has a 12V DC output that I'm planning on using to charge the batteries. My question is should I connect the generator directly to the battery bank or to the charge controller? Our current setup is wired like this:

PV Array -> Tri-Star TS-60 Charge Controller -> 1000Ah 12V Battery Bank (8 batts in parallel) -> 1200W/2400W Surge Inverter

I see the options as:
a) Connect the generator to the charge controller in parallel with our PV array
b) Connect the generator to the junction box where all 8 batteries meet
c) Connect the generator to a single battery in the bank.

(Would B and C end up in the same place because the batteries are in parallel??)


# Posted: 4 Sep 2011 12:49

For others' use here are some things I've discovered after experimentation...

It seems that 12V DC out on the AC/DC generator outputs exactly that, 12V +/- .05 This is not nearly enough to effectively charge a 12V battery. So anyone out there thinking like I was a few hours ago will be disappointed. You have to use a standard battery charger plugged into the AC side of the generator. Or have a generator that is intended to output enough volts to charge the battery.

Because of this, connecting up to the charge controller seems silly. So I just connected the battery charger directly to one of our batteries.

Also, so long as all the batteries wired in parallel are the same type (deep cycle, starting, marine etc) they can be charged in parallel by just connecting the charger to one of the batteries.

Hopefully this can help someone in a similar situation!

# Posted: 7 Sep 2011 04:19

We have AC generators, so we run our power to an inverter/charger, and it regulates the battery charge. If we want to charge one or two smaller batteries for a portable array, we use a standard battery charger.

Asking my EE hubby about your system, he says:

A is wrong, as it could push power into the solar panels not just the batteries.
B & C aren't safe or effective.

Batteries need to be charged at the proper rate and voltage (slightly higher than the battery voltage). You probably need an actual battery charger between the generator and the battery bank to regulate the charge so you don't burn out your generator, overheat your batteries or run your gennie forever for not much gain... unless your generator or inverter has a built-in charger. You don't mention what type of generator you're running so he can't make any recommendations for you.

Connecting the charger to one battery in the array will charge the entire array as long as they're the same and wired together properly.

# Posted: 7 Sep 2011 10:05

Yes, you need a battery charger that you power with the 120 VAC output from the generator.

The manual for that generator probably lists the 12 DC as an output. My Yamaha specifically ststea not for charging batteries. As mentioned above the output voltage is too low for charging. It is also too low an output to do anything by trying to feed through the CC.

A battery charger that has two modes, like a bulk chage rate and an absorb is best. They will charge faster and ttreat your batteries kindly once the charge reaches full or near to full.

Look for Iota brand.

# Posted: 9 Sep 2011 10:06

Get a charge controller.
We are charging our battery bank off the generator until we get our PV panels. Determine how many amps you need. You can get an IOTA 90amp charger for $300 +/-

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