Pb vs. LiFePO voltage sag

[broncobowsher]

I'm currently running 200 Ah of lead acid battery for a house battery. When I fire up the invertor and put a 100A load on the battery pack to cook up some breakfast I get enough voltage sag to kick off the diesel heater. Running big cables, short runs, the sag is in the battery itself not the wiring.

I'm in the planning stages to change over to LiFePO4 battery. How do those handle loads? Keep things a fair fight, 200 Ah battery as well. I might make other changes, but for comparison at this point anyway.

 

[DiploStrat]

Sag is, of course, a function of load, battery size, installation (as you noted), and yes, battery chemistry.

Assuming that the BMS on you lithium iron battery will pass the required amps, you will see less sag. And, you will almost certainly be starting from a higher resting voltage, as well.

Lead Acid batteries want to be kept above, say 50% charge for best life. Lithiums want to be kept BELOW, say, 80% charge. Some lithium batteries add extra capacity so that they can be "100% unitized." A good BMS will shut of the battery completely before it discharges too low. Most will do the same thing at the top end.

Soooo, assuming 200 Ah of lead, applying the 50% rule, i.e. 100 Ah usable, 200 Ah of lithium should give you between 150 and 180 Ah usable. (And some will yell at me for being so cautious. ) In any case, you should save a lot of weight as well.

 

[jonyjoe101]

There is a video on youtube "Lithium battery test versus Gel." where they test a 100ah lifepo4 versus a 100ah and 220ah lead acid on a 2600 watt heater load. The lifepo4 easily beat both batteries, the lifepo4 didnt sag too much (11.7 volts) during the test, both lead acids (100ah 10.3 volts/220ah 10.8 volts) were sagging at the beginning of the test with the low voltage warning light illuminating on the 100ah lead acid.
As far as usable power the lifepo4 provided 94ah (lasted 19 minutes) the 100ah lead acid 31ah (lasted 6 minutes) and the 220ah lead acid 73ah (lasted 13 minutes) before they gave up the ghost.
The lifepo4 they tested was a beast of a battery (super B) that can output 300 amps continuously, most lifepo4 available on amazon max out at 100 amp of continuous output. Lead acids have no output limit and are cheaper. The lead acid have to be fully charged all the time to get any type of good performance out of them,

 

[Alloy]

All batteries have a C (charge / discharge) rate. It's higher for Lithium. There's lots to read online.

Lithium needs to be heated but only if being to charge below +/-32F


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[john61ct]

LFP cells of good quality while very healthy, will hold voltage from sagging pretty darn well under a 0.5C continuous load

for a few minutes

maybe even 10-15 minutes

But if you're planning on doing that regularly, best to double/triple your Ah capacity in order to reduce the C-rate.

Not just keeping voltage up, but getting good longevity.

Consider using wood or propane whenever possible.

 

[Peter_n_Margaret]

"Dot points".
Voltage sag (if everything else is OK) is battery chemistry. This is a significant limitation with LA. It can not keep up with the demand. BUT, it will recover if you reduce the load and smaller loads will be fine. There is no loss of total stored power caused by high voltage drop.

The so called 50% discharge limit for LA is an urban myth. In fact, it is quite safe to discharge a LA to 10.5V. BUT it must be fully recharged regularly to avoid loss of life. Discharging a Li to "flat" may be detrimental BUT it can be used long term without needing to be fully charged.

The most significant restriction to Lithium discharge rates will be the MBS, not the battery itself. My choice would be to choose an external BMS whose specifications are known rather than a "black box".
Cheers,
Peter
OKA196 motorhome

 

[broncobowsher]

Thank you. That was what I was hoping to hear.
I am planning on running a BMS that is 150-200% of my anticipated load.
I have actually switched from a microwave (very high power consumption) to a toaster over (only high power consumption) and have found the total watt hours consumed is nearly identical. But the food out of the toaster over is much better. I can cook a tin of biscuits on a highway rest area, try that with a microwave. No fire needed either. I can cook in a gas station parking lot if need be and nobody would know.
Back to planning the battery change. Thermal limitations are going to be interesting. Thinking of an insulated box and maybe a Peltier cooler/heater. The good part is it doesn't need a lot of cooling, I don't need to keep it 72°, just below 120ish. Run a Peltier backwards and it makes more heat than it consumes, being it is a solid state heat pump. At worst it is as effective as a resistive heating element.

 

[john61ct]

Ventilation should be all that's needed.

Insulation of the box only in extreme cold

 

[broncobowsher]

Ventilation should be all that's needed.

Insulation of the box only in extreme cold

You have not been in a vehicle that is parked in the sun in Arizona in the summer. Interior temps can easily break 150+° and there are sometimes contests for cooking an egg or cookies on the dashboard.

 

[john61ct]

Ventilation, as in exchange with the outside air.

 

[Joe917]

Pulling 100 Amps out of a 200 Amp/Hr Lead deep cycle battery bank is pretty brutal. You must be exceeding the battery manufacturers discharge rate.

 

[Peter_n_Margaret]

Pulling 100 Amps out of a 200 Amp/Hr Lead deep cycle battery bank is pretty brutal. You must be exceeding the battery manufacturers discharge rate.

Does no damage. The battery chemistry slows down the discharge rate until it catches up.
LA batteries come as high recombination and low recombination. The typical difference is the number and thickness of the plates. More plates = faster chemical reaction to accept charge or deliver power.
Every crank battery discharges at more than 200A for a while every time it is used. If the car does not start, the battery gives up for a while, but will recover if left standing while the chemical reaction catches up.
Cheers,
Peter
OKA196 motorhome

 

[broncobowsher]

Pulling 100 Amps out of a 200 Amp/Hr Lead deep cycle battery bank is pretty brutal. You must be exceeding the battery manufacturers discharge rate.

Just looked up the spec sheet. It is rated up to 3C for full capacity. I'm only drawing 0.5~0.6C, intermittently, as the toaster oven cycles for 10 minutes or so. Yes, 100A is a good pull, but not excessive.