Looking to upgrade/replace battery bank. Questions about AGM vs LiFePO4

RoySalisbury · September 2021

I'm looking to upgrade my solar system to increase capacity and life. Currently I run a 24v system using 16 ea 6v-400ah AGM batteries (38,400 wh).

I will be moving up tp a 48v system. This means to add more capacity to my battery bank I would need to get a minimum of 8 more batteries. And adding new batteries too en existing bank is not always advised.

So I am thinking that perhaps it might be time to upgrade battery chemistry at the same time and go with LiFePO4. They are more expensive, but on paper seem to be worth the cost. So I have a few questions that I hope some experts can answer.

1. DoD. I never let my AGM batteries get below 40% before thee generator kicks in. But most of the time they never go below 30%. With LFP I understand that I could almost got to 90% without issue. Aside form the lifespan issues for going over 50% in AGM, the voltage gets to a level that the inverter does not like either. With LFP is this still the issue?

2. Temperature. With AGM I need to get the battery room climate controlled to I don't go over about 80F. This helps with lifespan and charging. This also means that I need to run an AC or heater system in the room and that uses power. I am reading that the LFP batteries can handle up to 140F without issue or degradation. If this is the case then thats less power I need to store in the bank.

3. Cycle. AGM kept in good condition (temp, DoD, ect) can get about 1500-2000 cycles (5 or 6 years). LFP even when not temp regulated and 80-90% DoD can get about 4000-5000 cycles.

Thanks for taking the time to respond.

mike95490 · September 2021

2. Temperature. With AGM I need to get the battery room climate controlled to I don't go over about 80F. This helps with lifespan and charging. This also means that I need to run an AC or heater system in the room and that uses power. I am reading that the LFP batteries can handle up to 140F without issue or degradation. If this is the case then thats less power I need to store in the bank.

You don't want to go much over 100F with Li. A good working range is 50F - 100F

Much below 50, and you start to loose capacity because of lazy chemical reactions.

At 35F (32F is the kill point) you need to stop charging the battery because it will be fried if charged below 32F.
Li has little self-heating, there are flexible heater pads you can place under a heat spreader under the batteries.

Pay attention to the battery room and firewalls. you don't want burning battery fumes to get into your house. FLA batteries are more likely to leak acid than they are to burn, but Li can burn and they are tough to put out.

RoySalisbury · September 2021

You don't want to go much over 100F with Li. A good working range is 50F - 100F
Much below 50, and you start to loose capacity because of lazy chemical reactions.
At 35F (32F is the kill point) you need to stop charging the battery because it will be fried if charged below 32F.
Li has little self-heating, there are flexible heater pads you can place under a heat spreader under the batteries.
Pay attention to the battery room and firewalls. you don't want burning battery fumes to get into your house. FLA batteries are more likely to leak acid than they are to burn, but Li can burn and they are tough to put out.

The battery room is a completely separate structure (40 ft container w/10x10 climate controlled area) and has ventilation fans to exhaust out the battery box.

Even at 100F, that still less power needed for the A/C unit (would set it to heat/cool between 50/90 instead of always trying to maintain 78-80). Basically that would have the unit running very little and save A LOT of power. During the summer months I use about 5kw overnight just to keep the temp under control (the panels keep up during the day).

Roy

jtdiesel65 · September 2021

RoySalisbury said:

I'm looking to upgrade my solar system to increase capacity and life. Currently I run a 24v system using 16 ea 6v-400ah AGM batteries (38,400 wh).

I will be moving up tp a 48v system. This means to add more capacity to my battery bank I would need to get a minimum of 8 more batteries. And adding new batteries too en existing bank is not always advised.

So I am thinking that perhaps it might be time to upgrade battery chemistry at the same time and go with LiFePO4. They are more expensive, but on paper seem to be worth the cost. So I have a few questions that I hope some experts can answer.

1. DoD. I never let my AGM batteries get below 40% before thee generator kicks in. But most of the time they never go below 30%. With LFP I understand that I could almost got to 90% without issue. Aside form the lifespan issues for going over 50% in AGM, the voltage gets to a level that the inverter does not like either. With LFP is this still the issue?

2. Temperature. With AGM I need to get the battery room climate controlled to I don't go over about 80F. This helps with lifespan and charging. This also means that I need to run an AC or heater system in the room and that uses power. I am reading that the LFP batteries can handle up to 140F without issue or degradation. If this is the case then thats less power I need to store in the bank.

3. Cycle. AGM kept in good condition (temp, DoD, ect) can get about 1500-2000 cycles (5 or 6 years). LFP even when not temp regulated and 80-90% DoD can get about 4000-5000 cycles.

Thanks for taking the time to respond.

For DOD, the longevity for LFP is said to be 10-90%. But if you are off grid, a more practical range might be 20-90% so you have some capacity left over in case generator doesn't start, etc. The voltage levels for the prismatic diy cells range 2.5-3.65. So for 24v 8 cell string 20v to 29.2v min to max. Operaitng range around 23-28.2v (will vary on setup). Voltages will vary depending on the exact battery in use.

IIRC desired temp for LFP was 77f. Each battery should have a spec sheet.

For cycles, it's whatever the manufacturer specs. The newer diy primatics spec at 6000 cycles, but take that with a grain of salt. That's also at lower discharge rates..which may be fine for off grid.

Marc Kurth · September 2021

Newer designs are using an internal Mylar wrap that is used to heat the cells. When charging power is applied to a cold battery, the BMS diverts the power to the wrap until the temperature rises to a safe level.

The Depth of Discharge curves will show you the impact of going into deeper discharge ranges. There is not some sort of cliff that destroys the batteries below 50% - it is a curve that all reputable manufacturers publish. Going deep occasionally is perfectly OK within reason. Just like any other product, not all Lithium or AGM batteries are the same.

The trick is find the balance of battery bank size/DOD/cost. With a shallow average DOD, a lead acid battery bank can often provide a cost effective investment.

IMHO, the big advantage of Lithium is the ability to hover around low charge levels that would destroy flooded/AGM/gel cells. Of course that can be solved with more modules.........

Marc

Photowhit · September 2021

Marc Kurth said:

Newer designs are using an internal Mylar wrap that is used to heat the cells. When charging power is applied to a cold battery, the BMS diverts the power to the wrap until the temperature rises to a safe level.

Thanks Marc! Good to know!

Looking to upgrade/replace battery bank. Questions about AGM vs LiFePO4

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