Charging a 12v lifepo4 battery on an automotive charger

[RyanNicholas]

Hi everyone. This might be a bit of a noob question to ask, but you guys are generally pretty supportive for people on the learning curve, so here goes.

I've been operating for the last year on the battery I built, which is a 280ah 4s 12v with a BMS from Current Connected (just like the overkill solar ones). It's been working great... but the sun has been pretty low lately, and it has me curious about something.

I would love to be able to charge this battery occasionally on an external power supply, hopefully without having to disconnect anything. If I hook up, say, a standard 12 volt auto battery charger to it, will that work effectively and safely to juice me up? I realize that, when fully charged, the voltage on this battery is closer to 14 than 12 - so it's just had me wondering if trying to juice up on just 12 volts is enough. And, if it would indeed work, could I just hook it right up to the main access terminals with everything else still connected to them? Since I'm not set up like an RV for 'shore power' connection, it's doing what I'm suggesting is a viable option, it would be super nice to be able to hook up a trickle charger overnight at my buddy's house while I'm parked overnight ( without having to completely lose power in the meantime). My amp hours have dwindled over the last couple of days, I just installed a diesel heater, and we're expecting snow this weekend.

Thanks in advance for any helpful advice!

 

[codfish]

Just about any 12v charger with an AGM or Gel setting should work fine for the occasional top up. (Gel preferred)
They are not suggested as a long term solution since they don't handle float in a LFP friendly fashion and they don't allow fine tuning of the top voltage. For for occasional charge it should be fine.
A charger with a traditional Flooded setting may overcharge the battery causing the BMS to disconnect.

 

[RyanNicholas]

Just about any 12v charger with an AGM or Gel setting should work fine for the occasional top up. (Gel preferred)
They are not suggested as a long term solution since they don't handle float in a LFP friendly fashion and they don't allow fine tuning of the top voltage. For for occasional charge it should be fine.
A charger with a traditional Flooded setting may overcharge the battery causing the BMS to disconnect.

Just as I posted this, I realized that since the isolator I have running from under the hood of my van back to my solar battery is clearly safe to run, with everything still connected to the battery and even on long road trips when the battery is already fully charged, that hooking up any other high amperage charger is probably about the same. And it seems the BMS is fully capable of cutting off power when necessary. I don't understand the different modes (AGM, Gel etc), or what "float" means. Why would the BMS disconnect like you're saying?

 

[Ampster]

I don't understand the different modes (AGM, Gel etc), or what "float" means.

The manufacturers come up with these terms. If you can reduce it to voltage and Amps it may be easier to understand. There are really only two stages when charging batteries, Constant Current and Constant Voltage. CC is often called Bulk and CV is Absorb. Float is a low current variation of CV that is typically only used on Lead Acid type batteries including AGM and Gel.

 

[Mike Jordan]

I have run automotive chargers dozens of times on LFP batts no issue. An automotive charger has never been my main charger but I have used semi regularly as a back up or travel charger

 

[RyanNicholas]

I have run automotive chargers dozens of times on LFP batts no issue. An automotive charger has never been my main charger but I have used semi regularly as a back up or travel charger

That's great to know. What do you think the maximum safe amperage is to charge one of these? I just called the auto parts store and they said they have a 12-volt charger with an 800 Peak amperage, which sounds insane to me but maybe that's safe?

 

[codfish]

Most home chargers have buttons or selector switches labeled "AGM" or "Gel" or "AGM/Gel" any of these settings would be fine. The terms are variations on the internal construction of the battery and correspond to the printed labels on most batteries. "Flooded" is yet another type and for a single night charge that would probably be fine as well. Some Flooded chargers might try to take the voltage high enough as the battery approaches full that the High voltage disconnect would be activated by the BMS. Not the end of the world, but worth avoiding.

An 800 amp charger sounds impossible for any reasonable price and wire size. Perhaps they were referring to a surge starting current that it can sustain briefly ( I have a charger that can do that for a second or so) but drops back to 50 amps to charge.

Maximum safe charge is "1C", or in your case 280 amps. The manufacturers often recommend 0.5 C for regular usage. In your 140 amps continuous. Ignore the "surge" rating.

 

[Mike Jordan]

That's great to know. What do you think the maximum safe amperage is to charge one of these? I just called the auto parts store and they said they have a 12-volt charger with an 800 Peak amperage, which sounds insane to me but maybe that's safe?

surrrrrrrrrrrrrrrrrrrrely they were referring to a 8oomah trickle charger. No car battery in the world could take 800 amps, and no 120v outlet in your house could power it

 

[RyanNicholas]

An 800 amp charger sounds impossible for any reasonable price and wire size. Perhaps they were referring to a surge starting current that it can sustain briefly ( I have a charger that can do that for a second or so) but drops back to 50 amps to charge.

Yeah, that's kind of what I figured.

 

[RyanNicholas]

OK, so if I use something like this, do these numbers mean that 20 amps is what I can expect on a consistent overnight charge?

 

[RyanNicholas]

surrrrrrrrrrrrrrrrrrrrely they were referring to a 8oomah trickle charger. No car battery in the world could take 800 amps, and no 120v outlet in your house could power it

Turns out she was telling me about a jump starter, not a charger.

 

[codfish]

Well I've never seen some of those particular advertising terms, but yes, I'd say 20 amps. The spec also says it has built in "timers" of 15 hours for LFP, and 72 to for Lead. I had a Battery Tender (different model) and I found I had to unplug and replug it to restart the timer every 12-15 hours. It takes a lot of hours to fill a 280ah battery.

 

[DJSmiley]

A modern auto charger will mostly work, but it usually lacks a proper LFP profile

You can use GEL/AGM/Whatever is available to see if it matches, but on a lot of them the float is higher than recommended for LFP.

Important is to check how the charging curve is, especially on chargers with over 3 stages.

Multi-stage chargers which periodically do a desulphation: That's 15V+. Your BMS will trip, reset, trip, reset during that stage if you're lucky, or worse. Disable that. If you can't disable that, don't use.

And be very carefull with old legacy transformer based chargers! They are not regulated at all and basicly just a transformer with some diodes. Does work for lead acid, but they output a wayy to high voltage. They basicly produce a rectified sine wave. While the measured voltage is 12V or so, the peak of the sine is sqrt(2) times this voltage, so thats approaching 20V on a 13.8V charger.

This is probably the reason @Will Prowse fried one of his BMSses in the past

I would just get a decent charger suitable for LFP.. Especially when charging overnight there is no need to push massive amps. Even 20A at 10 hours still charges 200Ah.. I think thats fine enough since you're likely not draining the battery to 0% SOC.
And with a overnight stay including the evening its more like 12+ hours to charger.
<30A chargers aren't that expensive.

I like the Victrons, but even cheaper is a Meanwell one (I use it myself). The PB-360P-12 is a 24A charger and is <$100
(I only would recommend adjusting it down a little to 14.2V or so)

 

[RyanNicholas]

Well I've never seen some of those particular advertising terms, but yes, I'd say 20 amps. The spec also says it has built in "timers" of 15 hours for LFP, and 72 to for Lead. I had a Battery Tender (different model) and I found I had to unplug and replug it to restart the timer every 12-15 hours. It takes a lot of hours to fill a 280ah battery.

At 20 amps, plugging it in for 10 hours should juice it up to about 200 amp hours, right? Since the battery is already partially charged, I'm hoping that plugging it in now means topping it off by morning.

 

[codfish]

At 20 amps, plugging it in for 10 hours should juice it up to about 200 amp hours, right? Since the battery is already partially charged, I'm hoping that plugging it in now means topping it off by morning.

I agree if it actually produces 20 amps. I've just never seen the term "20A Battery Boost" and don't know if they mean it will maintain 20amps until the charger enters Constant Voltage(CV) stage. If the "1.25A Charger/Maintainer" kicks in then you are not going to get much overnight.

 

[RyanNicholas]

A modern auto charger will mostly work, but it usually lacks a proper LFP profile

You can use GEL/AGM/Whatever is available to see if it matches, but on a lot of them the float is higher than recommended for LFP.

Important is to check how the charging curve is, especially on chargers with over 3 stages.

Multi-stage chargers which periodically do a desulphation: That's 15V+. Your BMS will trip, reset, trip, reset during that stage if you're lucky, or worse. Disable that. If you can't disable that, don't use.

And be very carefull with old legacy transformer based chargers! They are not regulated at all and basicly just a transformer with some diodes. Does work for lead acid, but they output a wayy to high voltage. They basicly produce a rectified sine wave. While the measured voltage is 12V or so, the peak of the sine is sqrt(2) times this voltage, so thats approaching 20V on a 13.8V charger.

This is probably the reason @Will Prowse fried one of his BMSses in the past

I would just get a decent charger suitable for LFP.. Especially when charging overnight there is no need to push massive amps. Even 20A at 10 hours still charges 200Ah.. I think thats fine enough since you're likely not draining the battery to 0% SOC.
And with a overnight stay including the evening its more like 12+ hours to charger.
<30A chargers aren't that expensive.

I like the Victrons, but even cheaper is a Meanwell one (I use it myself). The PB-360P-12 is a 24A charger and is <$100
(I only would recommend adjusting it down a little to 14.2V or so)

Just now saw this comment. A lot of what you said is Greek to me, but I just hooked up the battery tender one I posted the picture of above and it has a lithium settings so I'm guessing it's probably fine. I'm also curious about ones like this one on Amazon ( picture below)... Seems they are specifically designed for lithium batteries, and also a lot smaller than regular 12 volt Chargers.

 

[RyanNicholas]

How about something like this?

 

[codfish]

That should work fine for your overnight, occasional charging. But... remember don't charge if the battery will get below 32F overnight.

Long term one of the Victron Smart chargers (more $$$) can shutoff charging automatically based on a low temperature cutoff.

 

[RyanNicholas]

Well, since I just installed this amazing little diesel heater, I'm not concerned about things getting too cold in the van ?
Although it has also increased my power consumption, contributing to the need for extra charging (in addition to the weather).

 

[RyanNicholas]

I agree if it actually produces 20 amps. I've just never seen the term "20A Battery Boost" and don't know if they mean it will maintain 20amps until the charger enters Constant Voltage(CV) stage. If the "1.25A Charger/Maintainer" kicks in then you are not going to get much overnight.

Damn. Looks like this thing will maintain a 20 amp charge for maybe 10 minutes, then it drops to around 5 amps for some reason. Lame.