Absorption stage "timing"

[Don B]

I'm upgrading my AGM battery to a Battle Born LiFePO4, and must upgrade my RV's charging system too (it can't get up to 14.4v), but I'm really trying to understand why even the most expensive chargers that'll do lithium (Victron Multi, Magnum) do NOT have an external shunt that tells the computer how many amps are going into the battery, so you can set a "Full Amps" setpoint (sometimes denoted as "Tail Current") so the absorption stage can be only what the battery needs to get fully charged. Instead I'm told that a timing algorithm is used to determine when the absorption voltage (14.4) gets dialed down to the float voltage (13.6). That's fine if the battery's SOC is almost zero, but if the SOC only goes down to 80%, and you plug back into shore power, unless I'm mistaken, that absorption time period will occur again, but this time the battery can be sitting at the absorption voltage (14.4) for way longer than it needs to. Am I correct in assuming that, unlike a SLA battery, with a LiFePO4 battery, this will not hurt the battery? And if so. why can't it stay at 14.4 indefinitely? (The charger needs to output something for my 12v loads). Meaning, why does there have to be a float mode... why not just stay in the constant voltage mode of 14.4v?

Any insights would be greatly appreciated, so I don't remain pissed that a $1200 charger doesn't have an external shunt like my 12 yo $400 SLA charger did that had a "Full Amps" setting.

 

[John Frum]

I'm upgrading my AGM battery to a Battle Born LiFePO4, and must upgrade my RV's charging system too (it can't get up to 14.4v),<snip>

What is the highest voltage it can supply?

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[PHoganDive]

It's a good idea to use a DC to DC charger for charging lithium batteries from an automotive alternator. Lithium batteries are low enough internal resistance that the alternator can toast itself trying to charge it directly. The DC to DC charger will limit charge current and keep the alternator from cooking. You can also set the charge voltages of the DC charger separate from the automotive voltage regulator, so they are correct for a lithium battery.

 

[RCinFLA]

You are correct in that timed absorb charge is usually repeated every cycle

I would be a bit careful with an old lead-acid charger on a LiFePO4. Also be careful of low end discharge voltage compared to lead acid. LiFePO4 drop quickly below 12.8 v although the BattleBorn should cutout preventing over discharge and should also cutout if overcharge voltage is attempted.

On that old charger I would set absorb voltage at 14.0v and float at 13.6 v. Bulk time doesn't need more then 10-15 minutes and it won't hurt battery repeating it on a short recharge cycle. You will get over 97% recharge capacity and avoid 'whoops' damage.

Make sure your charger doesn't self trigger an equalization cycle that can take it above 15 vdc. Some 'smart' lead-acid chargers auto trigger an equalization cycle if the rate of recharging voltage rise doesn't fit their expectation profile. Some Schumacher auto battery chargers do this without any knowledge of battery AH size.