diy solar

diy solar

Advice on LiFePo4 Storage Voltage.... Off Grid System

RiffRaff

New Member
Joined
Feb 6, 2021
Messages
4
I am looking for advice on LiFePO4 battery storage voltages for a system that will get intermittent use during the summer and limited use during the winter.

My family owns an off grid camp in Northern New England. I have assembled components for a system using a 24V lithium iron phosphate battery. I have read several postings that suggest that these batteries should not be stored fully charged. One posting suggested that since this chemistry has very little internal discharge during storage that the charging 'float' option should be disabled or set to a voltage low enough to eliminate charging.

We have an integrated charger/inverter. The bulk charge cut off is set at 28.2 and the float is set at 27.0. The low voltage shutoff is set at 24V. I have a battery heating system for the cold weather.

This system will get used a dozen weekends between May and October. Maybe a 2 or 3 times during the winter months. We have generator back up if needed.

My plan has been to just let the system run whether the camp was being used of not. The panels would maintain the battery at full charge year round. There is a minimal power need to run the inverter. In the winter, if it gets cold enough, there will be some load on the system to keep the batteries warm

My question is.... If storage at 'full' charge is detrimental, should I develop a plan to discharge the battery down to say 26 Volts, no load, when we leave the camp and shut the system down??

Thanks in advance for any suggestions.
 
Short answer:
No need to do.
What you can do is lower the charge voltage the last day you are there.

90% full is perfect enough. (93% seems to be the sweet spot)
Your system is operational, so it automatically will discharge the cells to the " new normal" and with minimal power usage during night, it hardly needs to recharge during day.

What are you winning with this method? More cycles.
And if you don't..
Not less.
Still the 2000 you signed up for when you bought them.
With 93% max charge it seems 4-5000 cycles is reasonable target.

There will be more imbalance at 93%, not a problem for the winter but something you might want to set to 100% again on the last winter trip so the system have time to top-balance.

It probably adds about 1000 cycles this way.

Storage is not used at all.
From your information, things are operational, like some heating to prevent freezing.
That makes it a more standby setup, like most people have for their emergency power systems.
They don't do anything with it, unless there is a power failure :)
 
Short answer:
No need to do.
What you can do is lower the charge voltage the last day you are there.

90% full is perfect enough. (93% seems to be the sweet spot)
Your system is operational, so it automatically will discharge the cells to the " new normal" and with minimal power usage during night, it hardly needs to recharge during day.

What are you winning with this method? More cycles.
And if you don't..
Not less.
Still the 2000 you signed up for when you bought them.
With 93% max charge it seems 4-5000 cycles is reasonable target.

There will be more imbalance at 93%, not a problem for the winter but something you might want to set to 100% again on the last winter trip so the system have time to top-balance.

It probably adds about 1000 cycles this way.

Storage is not used at all.
From your information, things are operational, like some heating to prevent freezing.
That makes it a more standby setup, like most people have for their emergency power systems.
They don't do anything with it, unless there is a power failure :)
Thanks.

There is a lot of information on these batteries.... bulk charging voltage, charge cut off voltage, 100% charge, etc that seems to have a lot of 'personal opinion' as opposed to fact.

If the system runs correctly, I am certain there will be a couple of generations of new technology before the battery wears out.

Thanks again.
 
If the cabin use no power when there is no people there, you may just shut it off after full charge, it will save you some cycle life on that, but do check your BMS idle consumption, if you decide to shutoff the system when leaving the cabin, it is good to disconnect the BMS also.

if you decide to leave it on, just make sure you don't always float the battery at max voltage then everything should be fine.
 
Back
Top