Testing: Lead and lithium (LFP) battery banks in parallel

wattmatters said:

I would not do this with flooded LA. The voltages they need are too high, but with these Enersys SLA unit, it seems to work well.

The SLA are specified to be charged at 2.35-2.40 V/cell, which in 48V form is 56.4 V - 57.6 V. As our temps are generally pretty warm I normally use the lower end of 56.4 V, which suits the LiFePO₄ perfectly (average of 3.525 V/cell).

Float is specified at 2.27-2.29 V/cell which is 54.5 - 55.0 V, again that's equivalent to 3.40 - 3.44 V/cell for LiFePO₄.

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Well I have been running a Costco CG2 FLA bank in conjunction with my LFP bank for 5 years now. Although when I started many said it should not be done...... It has worked great as among other things, load sharing under heavy load and UPS duties. Now that I have added 300 AH of LFP, I will sometime this summer move them to the solar shed where they can charge EGO batteries and provide lighting and of course dropping tongue weight on the trailer will be a nice perk.

grizzzman said:

Well I have been running a Costco CG2 FLA bank in conjunction with my LFP bank for 5 years now. Although when I started many said it should not be done...... It has worked great as among other things, load sharing under heavy load and UPS duties. Now that I have added 300 AH of LFP, I will sometime this summer move them to the solar shed where they can charge EGO batteries and provide lighting and of course dropping tongue weight on the trailer will be a nice perk.

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That is curios. What voltage is your bank?

sunsurfer said:

That is curios. What voltage is your bank?

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It is a 12 volt system.

grizzzman said:

Well I have been running a Costco CG2 FLA bank in conjunction with my LFP bank for 5 years now.

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How does the LiFePO₄ manage with the higher voltages FLA needs to sustain?

Or do you have a device to switch banks in/out as needed?

wattmatters said:

How does the LiFePO₄ manage with the higher voltages FLA needs to sustain?

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Once in a while I disconnect the LFP bank and let the trailers converter do it's thing.

Btw @wattmatters great test work good science here.
FWIW IIRR AGMs do not like large charging currents dumped into them > 0.1 C for 100 Ah - its a chemistry thing, also volts abov 14.4V gassing?

So putting AGM//Lifepo you need to regulate charging to limit inflow to the AGM

Also AGMs need prolonged absorption for several hours after top cut off at 13.6

I am thinking that blocking diodes are needed and AGMs and Lifpos are fed into each behind the diodes (I am looking at mobile golf cart types stuff). My only interest here in parallel operation is to try out the Lifepo with the AGM as a changeover back up (as its already in place.)

Boron said:

Btw @wattmatters great test work good science here.

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Thanks but science it is not. Just data to be treated with due caution. Real science is far far more rigourous.

Boron said:

FWIW IIRR AGMs do not like large charging currents dumped into them > 0.1 C for 100 Ah

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I have 400 Ah of SLA @ 48 V so 0.1C is ~ 2kW so I think I'm fine on that front. The hybrid bank doesn't charge any faster than ~3.6 kW and that's while the LiFePO4 are taking the vast bulk of the charge.

And my SLA has a recommended minimum charge current of 0.1C when used for cycling. They are quite capable of handling more (not that mine ever do, with the exception perhaps of a return from a rare deeper discharge and this recovery can be a controlled event).

Boron said:

its a chemistry thing, also volts abov 14.4V gassing?

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I charge to 56.4 V which is equivalent to 14.1 V for a 12 V SLA battery or 2.35 V/cell. That's within the recommended charging voltage specs for my SLA batteries.

Only time I exceed that is the occasional controlled brief foray at higher voltage (57.6 V max) to recalibrate the LiFePO4 BMS SOC (that's an average of 3.6 V/cell for the LiFePO4). Charge currents are very low during such an operation and the voltage is still within the SLA charging specs (2.4 V/cell for the SLA).