LiFePO4 vs lead acid: performance in low SoC

[mrflux]

I have a 300Ah lead acid battery bank. But for unknown reasons, it was undercharged and is now damaged, it seems.
I'm therefore starting to consider what I might replace it with in the future.

I'm using the battery bank in my motorhome, which is charged with 440w solar panels / MPPT charger.

It seems to me that the lead acid batteries are easily damaged if stored at a low SoC (e.g. 30-50%) for several days (e.g. bad weather for several days).

Is this also a problem for LiFePO4? Or can they easily be discharged to 20-30%, charged to 40-50%, then discharged again to 20-30% for several days, without affecting the total amount of life cycle amp hours? (e.g. 200 Ah x 2000 cycles x 80% discharge = 320kAh cycles?)

Are there other problems relating to bad weather/undercharging/slow charging etc. that I should be aware of?

Also, am I correct to assume that the average discharge voltage is 12.3 for lead acid and 12.7 for LiFePO4?

Thanks!

[Kleinstein]

The Li cells have less problems with staying at low charge state for longer time. It is only the very low end (e.g. < 1...5% charge) that could be a problem. But once you go below a certain lower limit, they may not be safe to be used, so you absolutely have to avoid going too low. Partial charging is no problem at all - they even like is best to always stay in the 20-80% range, going very high or very low is some stress to the cells. So if you don't need the last bit of capacity one could stop at 90% and get a longer life (more cycles).

One other thing LI cells don't like are high temperatures. This could be problem in a car / motor home.

There is another downside: the batteries or often considered dangerous load - so there can be restrictions on how much you allowed to carry in you car without a special permit / warning signs.

[DavidMenting]

LiFePo4 can take some abuse. It depends on the specific cells you want to use. The A123 cylindrical 26650 LiFePo4 cells, for instance, are specified for a lifetime of more than 1000 cycles at 100% depth of discharge.

(From: http://www.a123systems.com/Collateral/Documents/English-US/A123%20Systems%20ANR26650%20Data%20Sheet.pdf)

In general, LiFePo4 batteries are very nice replacements for lead acid batteries. Seeing that you can run LiFePo4's down to 0% you don't need as large a battery bank as with lead acid batteries.

[tautech]

I have a 300Ah lead acid battery bank. But for unknown reasons, it was undercharged and is now damaged, it seems.
I'm therefore starting to consider what I might replace it with in the future.

I'm using the battery bank in my motorhome, which is charged with 440w solar panels / MPPT charger.

It seems to me that the lead acid batteries are easily damaged if stored at a low SoC (e.g. 30-50%) for several days (e.g. bad weather for several days).

While you've given us some basic info, we need more to make an informed recommendation.

Are your existing FLA cells a Deep Cycle variety ?

Your charging methods/systems seem inadequate IMHO and at least you should add additional charging facilities that should take the form of some or all of the below:
1. HD Mains charger for times you are connected to mains in the likes of camping grounds and motorhome parks.
2. House vs Backup charge controller to make full use of the alternator in your motorhome while traveling.
3. Small gasoline inverter style genset to power #1 when stationary or in poor weather. (low solar output)
4. Better battery state monitoring.

In addition to the above some consideration should be given to the installed utilities in an effort to reduce battery loading: LED lighting, higher efficiency inverters, lower consumption appliances (gas fridge) etc.

All these recommendations come from experience I've had helping my uncle fit out a large caravan the he and aunty would live in for at least 6 months every year while they travelled in their retirement.

[mrflux]

I've collected a lot of data in an spreadsheet, and come to the following pricing averages:

EUR/Ah
 - Cheap lead-acid (200 cycles): 1.6
 - AGM lead-acid (400 cycles): 2.3
 - Premium AGM (800 cycles): 3.2
 - LiFePO4 (2000 cycles): 5.86

But when adjusted for cycles, the LiFePO4 is a clear winner, with the only disadvantage being up-front capital cost for a battery that must be used for 5 years to recover the investment:

EUR/1000 Ah cycles
 - Cheap lead-acid: 8.13
 - AGM lead-acid: 6.65
 - Premium AGM: 4.51
 - LiFePO4: 2.93

And when you additionally consider that the effective Ah available in lead-acid are only half, the prices above actually double.

In addition, LiFePO4 benefits from:
 - very fast charging
 - low SoC doesn't harm the battery
 - higher discharge voltage means more Wh
 - smaller volume and weight for equivalent



My batteries are cheap deep cycle lead acid.

Circumstances:
- Full-time boondocking with very rare access to 230VAC. I'd rather disconnect load for a few days and let the MPPT charge the batteries completely than pay "docking" fees.
- Alternator provides up to 80A, but as voltage rises, the lead acid batteries are unable to absorb the current (needs higher voltage for fast charging, but even at 15.5V they only absorb 3A each, despite being at 1.22 specific gravity, probably because they're sulphated)
- Generator is not an option: too bulky, too expensive, too noisy, too inefficient (fuel/kWh)
- Gas fridge sucks for true boondocking: no leveling is possible

Thanks for the advice though.

The problem is that the lead-acid batteries simply don't absorb well enough.
Also, since I drive daily, using the alternator for charging would recover 70A x 20min = 23Ah (20Ah effective, maybe)

[tautech]

My old knackered batteries are cheap deep cycle lead acid.

Circumstances:
- Full-time boondocking with very rare access to 230VAC. I'd rather disconnect load for a few days and let the MPPT charge the batteries completely than pay "docking" fees.
- Alternator provides up to 80A, but as voltage rises, the old knackered lead acid batteries are unable to absorb the current (needs higher voltage for fast charging, but even at 15.5V they only absorb 3A each, despite being at 1.22 specific gravity, probably because they're sulphated)
- Generator is not an option: too bulky, too expensive, too noisy, too inefficient (fuel/kWh)
- Gas fridge sucks for true boondocking: no leveling is possible

Thanks for the advice though.

The problem is that the old knackered lead-acid batteries simply don't absorb well enough.
Also, since I drive daily, using the alternator for charging would recover 70A x 20min = 23Ah (20Ah effective, maybe)

Some comments embedded above.
Which brings me to ask, how is your 300AH bank configured ?
If there's multiple batteries, then your calculations based on: but even at 15.5V they only absorb 3A each are plainly incorrect.
My uncle too resisted the need for a genset, that cost him a pair of DC batteries and having learnt the lesson promptly went and bought a Honda inverter genset which BTW are incredibly small, quiet and little bigger than a 9kg LPG bottle.
The fridge he had would run on LPG gas, 230VAC or 12V, he used it ao all depending on his location, no sun or mains, use gas.

[splin]

Whilst Lifepo4 cells have many advantages over lead acid, one important disadvantage is that unlike the latter, Lifepo4 (and Li-ion) cells must not be charged below 0C.

That means you *must* either keep them above freezing in the winter or ensure that they cannot be recharged below zero.