Lithium battery/AGM charger....

[Lee697]

Hi guys....
I need some expert advice, please.
We recently bought a used 4WD, which has a pretty new lithium 120Ah battery as a fridge/accessories battery, and a DC-DC charger in the engine bay.
The battery -
https://au.adventurekings.com/kings-120ah-lithium-lifepo4-battery-quality-integrated-bms-2000-plus-cycles-long-life.html
The charger -
https://www.tjm.com.au/tjm-12-24v-25a-dc-battery-charger-032tjmdc25a

There is also a solar panel on the roof with a charge controller - I can only assume the DC-DC charger charges the Li battery.
The charger doesn't have a Li mode - it is set on AGM at the moment.

Is this safe? Is it OK for the battery?
Should I pull out the charger and buy a dedicated Li one?
I wonder if the previous owner may have had an AGM in the back, and upgraded it with a direct swap. The charger certainly looks professionally installed.

Thanks for any advice!!

[DavidAlberto]

Hi guys....
I need some expert advice, please.
We recently bought a used 4WD, which has a pretty new lithium 120Ah battery as a fridge/accessories battery, and a DC-DC charger in the engine bay.
The battery -
https://au.adventurekings.com/kings-120ah-lithium-lifepo4-battery-quality-integrated-bms-2000-plus-cycles-long-life.html
The charger -
https://www.tjm.com.au/tjm-12-24v-25a-dc-battery-charger-032tjmdc25a

There is also a solar panel on the roof with a charge controller - I can only assume the DC-DC charger charges the Li battery.
The charger doesn't have a Li mode - it is set on AGM at the moment.

Is this safe? Is it OK for the battery?
Should I pull out the charger and buy a dedicated Li one?
I wonder if the previous owner may have had an AGM in the back, and upgraded it with a direct swap. The charger certainly looks professionally installed.

Thanks for any advice!!

It does look likely the previous owner just swapped the AGM for lithium and left the charger as-is.

The pack's internal BMS acts as a fail-safe, so nothing dangerous is going to happen. It's just not the ideal setup. The AGM profile isn't tuned for LiFePO4, so it won't fully top off the pack or give the BMS enough time at high voltage to run cell balancing. Proper move is to drop in a lithium-specific DC-DC.

PS: I'm no expert, but hopefully this info helps 

[Geoff-AU]

Yup, nothing bad will happen.

12V AGM charges via bulk/absorption at 14.4-14.8V, then 13.5-13.8V floating.

12V LiFePO4 charges to an endpoint of 14.4-14.6V.

Crucially, the voltage safety margin of LiFePO4 is quite big.  A LiPo/Li-Ion finishes at 4.2V per cell and anything above that is dangerous.  A LiFePO4 is fully charged at 3.65V but can be safely taken up to 4.2V as well - there's just no point because you don't get any more capacity and time spent at max voltage slowly degrades the cell.  Basically, there's plenty of voltage headroom with LiFePO4.

The AGM charge profile may sometimes try an absorption charge which involves leaving the charge voltage at 14.8V for a few hours.  The worst that'll happen is the LiFePO4 BMS disconnects itself and then you're sitting there with a full battery and a charger that's doing nothing.  As soon as you put a load on it the battery will come back.

[Lee697]

Thanks for the replies.... I'll sleep easier.... I might replace the DC-DC with a lithium specific one, but at $500 I'll leave it for now....

[radiolistener]

Yup, nothing bad will happen.

12V AGM charges via bulk/absorption at 14.4-14.8V, then 13.5-13.8V floating.

12V LiFePO4 charges to an endpoint of 14.4-14.6V.

I've seen cases where people charged brand-new high quality LiFePO4 batteries (just out of the box) with unsuitable chargers and without a BMS, assuming that "nothing bad would happen overnight" (those were his exact words), only to find the cells swollen like balloons the next morning, resulting in about a 500 USD loss. He was lucky to notice it in time and disconnect the charger before the cells exploded, because he was charging the battery inside his house.

It shows why relying on "it'll probably be fine" isn't a good decision.

[Lee697]

The charger has a 'GEL' mode, limited to 14.2V - would that be a safer option?

[radiolistener]

I wouldn't choose the charging profile simply based on whether it's labeled "AGM", "GEL" or "LiFePO4". What really matters is the actual charging algorithm and its voltage/current limits and thresholds.

Lead-acid batteries (including GEL types) are generally much more tolerant of charging-voltage variations and occasional overcharging, whereas lithium batteries are much less forgiving of even small charging mistakes. I'd check the battery manufacturer's recommended charging profile and charger recommendations.

[Lee697]

The battery is placarded with 'recommended charge voltage 14.6V, max recharge current 60A'.... the GEL cycle on the charger states constant current to 14.1V, then constant voltage 14.1V until current drops to 3.8A.... AGM same with voltage limits at 14.4V.....

[radiolistener]

The problem is that a brief summary like "CC to 14.1 V, then CV at 14.1 V" doesn't fully describe the charging algorithm.

Many lead-acid chargers, especially higher-end ones, include additional stages (desulfation, recovery, equalization, temperature compensation, etc.) that are perfectly acceptable and beneficial for lead-acid batteries, but could easily overcharge or damage a LiFePO4 battery.