Comparison of FLA and LiFePO4 capacity

[Rusty123]

I've seen many posts that compare the capacity of FLA and LiFePO4 batteries, usually in an attempt to identify the cost per AH for each type. Generally, the calculation assumes that the usable range of the FLA bank is between 50% and 80% SOC (30% net), and that the usable range of the LiFePO4 bank is between approximately 20% and 95% SOC (75% net). I can follow all that, but it seems to me that the comparison should be made based on the bank capacity at a normal rate of discharge, rather than at the 20 hour rate.

For example, a 450 Ah FLA bank (20 hour rate) is actually closer to a 760 Ah bank at a 3.2 Amp discharge rate* (close to what many of us would experience on a boat). This is due to Puekert, of course. However, since the LiFePO4 bank has a Peukert close to 1.0, it does not experience the same "growth" in capacity at lower discharge rates.

In other words, a 450 Ah FLA bank (20 hour rate) has a capacity of 760 Ah at a 3.2 Amp discharge rate, and 30% of that yields an usable capacity of 228 AH. To get the same usable capacity from a LiFePO4 bank, you would need a bank with a rated capacity of 228/75% = 304 Ah -- smaller than the 450 Ah FLA bank to be sure, but not dramatically so.

Is my logic sound?

(*these figures were extracted from a separate thread discussing Puekert effect with FLA banks.)

[downunder]

If you work your way through the Li for house banks thread you will find it explained.

http://www.cruisersforum.com/forums/...ml#post1419017

[Rusty123]

Originally Posted by downunder

If you work your way through the Li for house banks thread you will find it explained.

I am sure you are right, but that would take weeks

[deckofficer]

Originally Posted by Rusty123

I am sure you are right, but that would take weeks

Yep, it is a rather long thread. Your math might be a little off. Using a
Rolls spec sheet their model S-600 with a 450 a-hr rating at 20 hours has a 599 a-hr rating at 100 hours. Problem is if enjoying the good life with inverter usage for the microwave, coffee maker, and other high draw appliances then that same 450 a-hr battery (20 hour rate) is derated. A LiFePO4 cell gets its a-hr rating at the brutal 1 hour rate and leaving 20% SOC in the cell unlike the 0% SOC of a LA test. The big and heavy Rolls S-600 450 a-hr battery only has 126 a-hr at the 1 hour rate.

1 hour rate

Rolls S-600 @ 115 lbs gives 126 a-hr

(2) CALB SE130 (to match Rolls 6 volts) @ 20 lbs gives 130 a-hr

I know your marine loads won't be 1 C but look at the difference. A 115 lb LA battery will deliver 126 a-hr while a 20 lb Li will deliver 130 a-hr and still have 20% SOC remaining.

[Rusty123]

I think we are both saying that the relative advantage of LiFePO4 over FLA in terms of capacity varies with rate of discharge, with LiFePO4 providing more relative advantage as the rate of discharge increases, and less as the rate of discharge decreases. Therefore, a potential user needs to evaluate the merits of each at his or her expected discharge rate, which is probably quite different than the rate at which the battery rating is derived.

Of course, as you point out, capacity is not the only criteria for comparison. Others include weight, acceptance rate, cost, lifespan, and need for maintenance.

Thanks for the clarification.

[T1 Terry]

The other consideration is that just like flooded cell and AGM batteries, the capacity is at a given discharge rate. The capacity for most quality lead acid deep cycle batteries is C20 or 5% of the rated capacity read as amps, Li batteries are rated at 0.5C or 50% of their rate capacity read as amps. Both batteries will deliver more than their advertised capacity at a lower discharge rate but only an Li battery will deliver it's advertised capacity at double it's advertised discharge rate. Add to this the Li battery will maintain a higher voltage during the discharge than a lead acid battery and the all important watts calculation comes into play. The higher the voltage, the lower the amperage for a given delivery of watts, so the discharge rate from an Li battery will always be lower, that one is very hard to put into solid numbers over the discharge period, measuring the cut voltage is the next. If 12v under load is used as the bench mark the comparison becomes even further apart, the Li battery will maintain 12v at a light load until completely discharged, a lead acid battery of any type will not, the higher the load, the greater the gap becomes.

T1 Terry

[Rusty123]

It almost seems to good to be true, doesn't it? Like one step short of perpetual motion.

[mitiempo]

Originally Posted by Rusty123

It almost seems to good to be true, doesn't it? Like one step short of perpetual motion.

Not really. More like the fuel tank that is easily filled and good to the last drop. We have become so used to lead acid batteries and their characteristics it does seem wild. This link shows Maine's 400 AH LiFePO4 bank at 12.79 after a constant 100 amp draw for over an hour.

Batteries - What The Future Holds....... - SailboatOwners.com

Interesting post showing the future of this battery technology.

[senormechanico]

Having had a fair amount of cruising experience with both types,
I'd never go back to FLA. Even for free.

[Rusty123]

I've been looking into converting my bicycle to electric assist. LiFePO4 seems to be the hot ticket in that world too, probably as much because of the weight savings, as well as the electrical performance.

[T1 Terry]

Originally Posted by Rusty123

I've been looking into converting my bicycle to electric assist. LiFePO4 seems to be the hot ticket in that world too, probably as much because of the weight savings, as well as the electrical performance.

I just bought a couple second hand. One had wiring issues causing power motor operation but a quick review of the wiring soon located the problem and it goes quiet well. The other one, mine of course, had issues with a very narrow power assist window before the BMS shut every thing down. A strip down of the battery showed one group of paralleled cells completely flat while the others were still near fully charged. The cause, very poor quality spot welding of the link plates. A quick 20 mins cleaning and soldering all the link plates back on properly, a balance charge to get that cell back up to full, fitted a cell logger so I could monitor the cell voltages (a must have) and we were back in action.
For a 250w 24v front wheel assist the gain is quite amazing, no more struggling with a crook knee on take offs and getting off and walking at the first sign of a hill, just keep the pedals circulating and hit the power assist when needed and away it goes. Not much faster than a steady maintainable riding speed for the average fitness person but plenty fast enough to get around. The 20Ah battery pack is quite light, you can notice the added weight in the front wheel so it's not a stunt bike or anything like that, but it's brought a world of push bike riding back into the realms of reality for me so I'm real happy with the outcome.

T1 Terry