down sides to over-sized battery bank

**SunEagle** · 01-10-2022, 10:24 PM

Hello panderson03 and welcome to Solar Panel Talk

The downside to having more batteries then charging amps is that too little amount of charging amps can sulfate the battery plates and slowly make them die.

For a FLA type battery system it is best to have a charging current between C/12 and C/8 where C= the Battery system Amp hour value. Usually with more batteries then charging amps you can see a rate greater then C/12 which is harmful to the batteries.

So in short you can add more batteries but you also need to add more charging amps which usually means either more panels or maybe go to an MPPT type charger which is about 33% more efficient then a PWM type.

And finally having more batteries might make you wire more in parallel which is also bad because you can charge and discharge them unequally which again kills one or more of the batteries.

**panderson03** · 01-10-2022, 10:35 PM

Thank you very much for your help, SunEagle

**checkthisout** · 01-11-2022, 07:59 PM

Originally posted by panderson03

Thank you very much for your help, SunEagle

What do you mean when you say oversized?

Oversized for your loads or Oversized for what your panels can put out on any given day? Or both?

In your particular case and what I have done is the solar basically acts to keep the batteries topped off and run a security system when I am not there. When I am there I simply treat the place as if it were an RV without solar and run off the batteries until depleted and then run the generator which runs the whole place and charges the batteries back up then shuts down and you run on battery, repeating the cycle.

If you're running a backup generator, there are not really any downsides.

In fact, I would argue there zero downsides to a big battery bank regardless of your circumstances other than increased initial cost and increased maintenance (more battery cells) but I doubt you'll lose much water.

**SunEagle** · 01-11-2022, 08:15 PM

Originally posted by checkthisout

What do you mean when you say oversized?

Oversized for your loads or Oversized for what your panels can put out on any given day? Or both?

In your particular case and what I have done is the solar basically acts to keep the batteries topped off and run a security system when I am not there. When I am there I simply treat the place as if it were an RV without solar and run off the batteries until depleted and then run the generator which runs the whole place and charges the batteries back up then shuts down and you run on battery, repeating the cycle.

If you're running a backup generator, there are not really any downsides.

In fact, I would argue there zero downsides to a big battery bank regardless of your circumstances other than increased initial cost and increased maintenance (more battery cells) but I doubt you'll lose much water.

A battery bank that is slowly charged due to limited amps (due to pv or CC) can hurt them. Anyone that owns a battery system knows they also have to have a generator but you have to be there to charge the batteries. The OP was just looking into adding batteries without adding any charging amps. That is a Downside.

**checkthisout** · 01-11-2022, 08:37 PM

Originally posted by SunEagle

A battery bank that is slowly charged due to limited amps (due to pv or CC) can hurt them. Anyone that owns a battery system knows they also have to have a generator but you have to be there to charge the batteries. The OP was just looking into adding batteries without adding any charging amps. That is a Downside.

Hopefully he comes back with some numbers. I would love to see the capacity of batteries, panels and what he plans to run.

**SunEagle** · 01-11-2022, 08:53 PM

Originally posted by checkthisout

Hopefully he comes back with some numbers. I would love to see the capacity of batteries, panels and what he plans to run.

I agree. What the OP has provided is not enough to give them good direction on what to purchase to increase the system

**panderson03** · 01-11-2022, 11:21 PM

Originally posted by checkthisout

What do you mean when you say oversized?

Oversized for your loads or Oversized for what your panels can put out on any given day? Or both?

In your particular case and what I have done is the solar basically acts to keep the batteries topped off and run a security system when I am not there. When I am there I simply treat the place as if it were an RV without solar and run off the batteries until depleted and then run the generator which runs the whole place and charges the batteries back up then shuts down and you run on battery, repeating the cycle.

If you're running a backup generator, there are not really any downsides.

In fact, I would argue there zero downsides to a big battery bank regardless of your circumstances other than increased initial cost and increased maintenance (more battery cells) but I doubt you'll lose much water.

thanks for your advice! we are using SOK LiFEPO4 12V batteries. we have an Outback Flexmax 80 charge controller.
the idea is that our battery bank would be oversized for what our panels can put out.
that way, when we're away from the cabin for a week or 2, the batteries can receive and store an abundance of energy, for our use when we return.
thanks again for weighing in

**panderson03** · 01-11-2022, 11:23 PM

Originally posted by checkthisout

Hopefully he comes back with some numbers. I would love to see the capacity of batteries, panels and what he plans to run.

we've purchased 9 370 watt panels and 4 12V SOK LiFePO4 batteries. seriously considering purchasing another 4 SOKs

**panderson03** · 01-11-2022, 11:24 PM

Originally posted by SunEagle

I agree. What the OP has provided is not enough to give them good direction on what to purchase to increase the system

please let me know what other info is required. thank you for your help!

**SunEagle** · 01-11-2022, 11:39 PM

Originally posted by panderson03

please let me know what other info is required. thank you for your help!

How are you wiring those 12V batteries? Are they a 48v system where you have them wired 4 in series or some other voltage and have some of them wired in parallel? And what is the Amp Hour rating of your battery system?

**panderson03** · 01-11-2022, 11:46 PM

Originally posted by SunEagle

How are you wiring those 12V batteries? Are they a 48v system where you have them wired 4 in series or some other voltage and have some of them wired in parallel? And what is the Amp Hour rating of your battery system?

the 4 12V SOK's are wired in series to make 48V. at 207AH

**SunEagle** · 01-12-2022, 03:26 PM

Originally posted by panderson03

the 4 12V SOK's are wired in series to make 48V. at 207AH

Then with 3300 watts (9x 370w) and an 80 amp MPPT CC you actually may have to much charging amps for that battery system.

**panderson03** · 01-12-2022, 08:32 PM

Originally posted by SunEagle

Then with 3300 watts (9x 370w) and an 80 amp MPPT CC you actually may have to much charging amps for that battery system.

and so need more batteries?
thank you

**SunEagle** · 01-12-2022, 10:22 PM

Originally posted by panderson03

and so need more batteries?
thank you

Just be careful how you wire them into the system.

Also understand that adding new batteries to existing/older ones will bring down the health of the newer ones.

It sucks but sometimes purchasing an entire new set to meet the Amp hour rating needed is the best thing to do.

down sides to over-sized battery bank

down sides to over-sized battery bank

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