backchannel: response to “Can You Charge a LITHIUM Battery With a LEAD ACID Charger?” video

Some thoughts on this video.

from the video

@0:24 We are going to combine [in the discussion] the lead-acid and AGM together since their charging profiles are pretty similar

Well, they both require three-stage charging. But the AGM voltage setpoints will typically be lower and charge rate minimums/maximums higher.

@0:26 if you have something that will charge lead-acid you can go to AGM there’s no real concern there

Overvolting AGM with excess Vabs (as found in many SLA charging profiles) will damage AGM by permanent venting. Overcurrent can also outrun the batt’s recombinant abilities but most folks don’t have enough charging grunt to exceed AGM’s typical 0.33C max.

@0:46 … then we have the Float that’s getting it up all the way to 100 kind of slow

Short answer: if your bank is charging in Float stage it’s misconfigured or poorly designed.

Longer answer: The Float stage is a maintenance stage, intended to offset lead’s self-discharge after fully charging.

Float should only occur after successful completion of Absorption. With lead batteries this will be something like ≤0.02C charge acceptance at Absorption voltage (Vabs hereafter).

@1:12 we’re running 14.4, 14.6v [for LFP] depending on the charger

LFP will charge to 100% SoC at voltages as low as 13.6v (3.4Vpc). Especially in a home charging scenario where you aren’t trying to get the bank recharged in 2 hours or something.

@1:19 … you may have read… if you use that lead-acid charger you may not get the full capacity or full charge on your lithium battery. Let me graphically show you why that’s the case.

Except it’s not the case.

  • 4S LFP will charge to 100% SoC at voltages as low as 13.6v if we allow enough time for absorption.
  • 4S LFP will charge to 100% at moderate rates like 0.2C at ≥14.0v with no Absorption

Reality check: Will your lead-acid charger charge to ≥14v? If so, congratulations, it will charge your LFP to 100%. If you can extend the Absorption duration you can do it with ≥13.6v.

@1:28 [shows discharge graphs]

Why is a discharge graph being used? If one wanted to “graphically illustrate” then a charge graph is required. Unfortunately, such a graph would reveal how LFP actually charges and so would contradict the video’s claims.

@2:13 [adds typical lead-acid profile float line]… that’s where the battery gets up to 100%

It gets to 100% at the end of Absorption. The “float finishes the charging” misconception is taking the poster on a goose chase.

@2:23 … at 13.2v

Float setpoints are configurable on decent chargers. And generally speaking only Gel floats that low. AGM generally floats higher and FLA much higher.

@2:27 … that’s where you’re going to end up using a lead acid charger on a lithium battery.

No. It’s already charged to 100% by 14.0v (before Float even started).

that’s going to get you to 70, maybe 80%

NO. Charging LFP to 13.2v Vabs might get you into that range, but charging to 14v will get the bank to 100%. What you do with the float after that is your business.

A 13.2v quasi-float after full charging is not insane with LFP. It will allow the SoC to relax a bit to ease stress on the cells. 13.3v is also worth playing with.

@2:40 … Dakota … specifically referenecs using this….

Dakota (who does sell LFP chargers but not Pb chargers) is selling LFP chargers by spreading FUD.

It’s laughable for Dakota to say LFP chargers are requireed for longevity then tout a 1C charging rate.

2:49 to summarize this up… much higher voltage for lithium and lower for the lead acid.

Flooded lead acid typically requires 14.7-14.8Vabs, higher than LFP chemistry can tolerate.

@2:55 when you look at charge time you can really throw that current to the lithium battery..

We are conflating two ideas here: charge current and charge duration.

  • AGM and LFP have similar recommended charge rates, typically 0.2C. Yes, you can push LFP harder if needed; 0.5C is usually the recommended max charge rate.
  • the charging time comparison has more to do with lead’s lengthy Absorption duration when it’s accepting relatively little current.

poster comments

The BMS will manage the current going to the cells to slow the 0-20 and 80-100 charge rate.

The BMS will not manage the current in any normal sense. It will disable the current if problems arise (temp extremes, overcurrent, cell overvoltage).

Exception: exotic/$$$ setups like external BMSes networked with dedicated alternators running external regulators (like a Wakespeed). In that case the BMS could inform the Wakespeed of the amount of current it wants from the alternator.

You still need to get the battery to 100% for proper cell balancing. A lead acid charger will not do that.

Both of those points are demonstratably untrue.

Charging at high voltages in order to balance cells is like pulling up to every red light at 100mph so we can feel the ABS engage. It’s causing the problem in order to solve the problem.

If you want to see for yourself drop the Absorption voltage down to 13.6v and give it enough time to Absorp. Observe the cell balance during charging and upon completion. My delta is usually 4-10mV all the way up to 100%. Balancers rarely kick in because the cells stay in balance when charged with moderate current/Vabs..

If you have multiple batteries, you should have a bank for each one or you will end up with some imbalance.

/slaps forehead

Multiple batteries paralleled into one larger bank. Completely normal.

viewer comments

So if I don’t have a converter that handles lithium batteries, but the solar charge controller does have a lithium setting would the solar not top the battery off after the 80% from the converter?

Unless the converter floats ≤13.4v it’s likely to hold the LFP at excessive state of charge. I’d probably set the converter to a “dumb” 13.2v and let the solar do what it wants. Or since you’re on shore power if the converter is in play disconnect the solar and let the converter hold 13.2.

If my chassis battery “lead”in my RV is dead can I hit the boost button and get a charge from the lithium house batteries to start my engine?

yes, but I would not crank from the LFP. Let the batts equalize voltage for a few minutes, separate, then crank. more info on self-jumpstarting

Other sources claim a lead acid charger will damage your lithium battery because of the float mode slowly frying it. Is that true or not?

Float mode isn’t the problem. I mean you could “float” at 12.5v if you wanted. The problem is holding Li at high SoC (something of a problem) and/or high voltage (significant problem) for long periods.

In a shore power setting (charging Li trolling battery at home) no float is required at all since the Li self-discharge rate is insignificant. In an offgrid scenario (charging Li from solar) a float of ≤13.4v will let the panels support the loads while allowing the battery to relax from 100%.