Parallel LifePO4

[Lumpy Waters]

I've been looking to upgrade from my Goal Zero AGM Yeti 400 for a while and I recently pulled the trigger on a Renogy DCC30S dual input MPPT/DC-DC charger and two 12V, 20aH LifePO4 batteries, which were black Friday deals. Now, I'm thinking I may have jumped the gun on the 20aH batteries without thinking some things through. Since the DCC30S has a 30A charging current capability and the 20aH batteries are rated at 20A max charging current, I believe it would be safe to charge both batteries in parallel, but would I be setting myself up for trouble should the BMS on only one battery cut off, putting the other battery potentially in an overcurrent situation?

[saltydawg]

The batteries, if close enough, will self balance each other once hooked up together. Check the voltage on both, take the higher one and put a small load on it, and get it with in .2 or even better .1 volts. Then hook them up to each other and leave them for 24 hours, then take them apart and check the voltage, it should be the same. Once they are the same, either new or by being hooked up you can charge them as one. You will want to check them every month or so, and before the first use of the year and make sure they are still the same. That just checks to make sure you did not have a cell go bad.

Even if you do loose a cell the bms will shut of the charging in each battery if its too many amps or too much voltage.

You do know to store them at 75 % charge or so, correct?

And buy a meter with a shunt so you can track the charge %.

best 44 bucks you can spend.
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[saltydawg]

I just read the manual for your charger, it will only give 15 amps from solar and 15 amps from the tow vehicle. So under normal use I dont see it getting the full 30 amps. For the most part it will be only solar or only vehicle, unless your panel is exposed while driving and the battery is at the point where it can even take that much power.

Also make sure the temp sensor is between the batteries as it will prevent a below 32 degree F charge which can hurt the battery.

[Lumpy Waters]

The batteries, if close enough, will self balance each other once hooked up together. Check the voltage on both, take the higher one and put a small load on it, and get it with in .2 or even better .1 volts. Then hook them up to each other and leave them for 24 hours, then take them apart and check the voltage, it should be the same. Once they are the same, either new or by being hooked up you can charge them as one. You will want to check them every month or so, and before the first use of the year and make sure they are still the same. That just checks to make sure you did not have a cell go bad.

Even if you do loose a cell the bms will shut of the charging in each battery if its too many amps or too much voltage.

Thanks. I think I realize now the error in my thinking. But, can I assume that if there is too much resistance between the two batteries that they won't properly balance?

Also make sure the temp sensor is between the batteries as it will prevent a below 32 degree F charge which can hurt the battery.

You do know to store them at 75 % charge or so, correct?

And buy a meter with a shunt so you can track the charge %.

Great advice, thanks! If i were to arrive home with batteries at 100%, would I then want to deplete the charge down to 75% before buttoning everything up? This adds a an element of post-trip complication that I was intending to reduce by installing a fixed system in place of the portable Yeti. (I am currently using an older, kind of heavy Goal Zero AGM Yeti with a second outboard battery that gets lugged into and out of the camper every trip)

I just read the manual for your charger, it will only give 15 amps from solar and 15 amps from the tow vehicle. So under normal use I dont see it getting the full 30 amps. For the most part it will be only solar or only vehicle, unless your panel is exposed while driving and the battery is at the point where it can even take that much power.

As I understand it, with no voltage supplied from the vehicle battery/alternator input, the solar charge controller will go to the full 30A. However, I decided to buy a third battery just to have some redundant capacity in case one of them crapped out on me.

[saltydawg]

Thanks. I think I realize now the error in my thinking. But, can I assume that if there is too much resistance between the two batteries that they won't properly balance?

They will self balance pretty well, even with just a few ohms of resistance, sure you want to keep it low, under 5 ohms or so but thats easy

Great advice, thanks! If i were to arrive home with batteries at 100%, would I then want to deplete the charge down to 75% before buttoning everything up? This adds a an element of post-trip complication that I was intending to reduce by installing a fixed system in place of the portable Yeti. (I am currently using an older, kind of heavy Goal Zero AGM Yeti with a second outboard battery that gets lugged into and out of the camper every trip)

They are happiest being stored partial discharged, but 100 % wont kill em. I would once you get home and see they are 100%, disconnect the charging and turn on the lights as you unload, or use the water pump to drain the tank, or both. When done unloading shut it down. Thats why I installed anderson connectors on my batteries, I can just unplug em.

As I understand it, with no voltage supplied from the vehicle battery/alternator input, the solar charge controller will go to the full 30A. However, I decided to buy a third battery just to have some redundant capacity in case one of them crapped out on me.

Edit it loos like your correct even thou I just edited out my double down on being wrong. I would not worry about it, that what the bms is for, also you would need about 450 to 500 watts of solar to get 30 amps of charge anyway