Lifepo charging from car?

[Taylorhayis]

Gday!

I’m building a 4s lifepo portable battery box, and hstuck on charging.
I’m using a low voltage cutoff, and cheap bms and if I ram 14.6v in the charging ports it charges… but I’m aware a cc/cv setup is better.

The only option I can find for charging these is mains voltage based, but I want to charge from 12v in my car… my charging test is using a 300w buckboost to get the 14.6v from a 12v supply.

Is there any reason I can’t use a cheap and nasty solar controller (as long as it’s lifepo compatible) to charge my battery, and instead of solar - feed it 18v or so (from my buckboost) to charge the battery safely?

Otherwise it looks like extremely expensive dc/dc chargers are the go… or using an inverter to run a 240v lifepo charger ?

 

[Bud Martin]

What is Ah of the battery and what is the max charge current for your battery pack?
You will need charger that you can set charging current so you will not damage the alternator.

 

[Taylorhayis]

What is Ah of the battery and what is the max charge current for your battery pack?
You will need charger that you can set charging current so you will not damage the alternator.

Thanks for the response Bud!

24ah, made of 8ah cells. Surely the 300w max of the buckboost will keep it down? However im glad to hear that my idea of using a pwm solar controller didn’t sound too crazy…

 

[Short_Shot]

Thanks for the response Bud!

24ah, made of 8ah cells. Surely the 300w max of the buckboost will keep it down? However im glad to hear that my idea of using a pwm solar controller didn’t sound too crazy…

300w is a lot of current at 14.6v. Over 20 amps. Do you need to charge at nearly 1C rate?

Depending on your vehicle that might be an issue even having that much alternator left.

Never mind the wiring requirements.

If you must insist on going that route and you have the parts have at it, just watch your wiring.

 

[Supervstech]

300w is a lot of current at 14.6v. Over 20 amps. Do you need to charge at nearly 1C rate?

Depending on your vehicle that might be an issue even having that much alternator left.

Never mind the wiring requirements.

If you must insist on going that route and you have the parts have at it, just watch your wiring.

It is worse than that… the buck boost INPUT amperage will be over 25A @12V… odds are good the buck boost will be amperage input limited.

 

[Short_Shot]

It is worse than that… the buck boost INPUT amperage will be over 25A @12V… odds are good the buck boost will be amperage input limited.

I mean. Kind of. Your alternator typically isn't 12v though. Varying up to 14.x volts.

But it probably likely to approach 25a for sure.

 

[pollenface]

I used to use a small inverter with a 240vac lifepo4 charger, a cheaper means than a dc-dc charger.

 

[Taylorhayis]

I used to use a small inverter with a 240vac lifepo4 charger, a cheaper means than a dc-dc charger.

Why the crazy price for dc-dc when it costs nearly nothing to do it this way with so much unnecessary conversion?

 

[astropuppy]

I use a - cheap - hobby charger and charge my 50Ah lifepo4 portable battery box at 10 amps while driving. Only inconvenience is I have to restart the charger every-time I turnoff and restart the car. There are many 12v (source) hobby chargers available. 10amps is the most current I feel comfortable with through a cigarette lighter plugin. I actually prefer 5amp charge rate. Dometic makes a real expensive - car - charger for their refrigerator power pack.

 

[MisterSandals]

Why the crazy price for dc-dc when it costs nearly nothing to do it this way with so much unnecessary conversion?

I suspect it has more to do with managing the wildly varying voltage and amperage from the alternator than doing that actual conversion that makes them spendy. Also, managing your target battery charging while limiting the pull from the alternator which keeps you alternator from overheating (bad).

Not using a DC-DC charger can get expensive quick(er?).

 

[Halfwalker]

I have a similar situation - a 4S LiFePo battery in the car specifically to run the dashcam when parked. Plan is for a simple relay to connect to the main car +12V supply when the car is running (and able to provide power to charge the LiFePo). When parked the relay relaxes open, disconnecting the main battery. Straight connection is kinda meh, really need a proper LiFePo charger that can take 12V DC in.

The pack is a 4S/5P configuration of Headway 38120 8AH batteries, with a 35A BMS. Figure about 40AH capacity, should provide for days of dashcam parking time (good for airport parking for example).

As said here, DC-DC converters tend to be pricey. But using a solar charge controller seems to be the way to go. Not much $$, 10A/20A/30A versions available a-plenty, just pick one that will charge at an appropriate rate, within reason. I think a 20A one would suffice - not too much hit on the alternator. For the fun car I had already replaced the oem 100A alternator with a 200A one (and upgraded feed wire), so capacity to spare.

So, question. Any charge controllers come recommended ? Or, any to stay away from ? Something like this in the 20A or 30A version looks reasonable

https://www.amazon.com/ExpertPower-Controller-Auto-Select-Intelligent-Regulator/dp/B08ZK1NMS7

 

[timselectric]

I have a similar situation - a 4S LiFePo battery in the car specifically to run the dashcam when parked. Plan is for a simple relay to connect to the main car +12V supply when the car is running (and able to provide power to charge the LiFePo). When parked the relay relaxes open, disconnecting the main battery. Straight connection is kinda meh, really need a proper LiFePo charger that can take 12V DC in.

The pack is a 4S/5P configuration of Headway 38120 8AH batteries, with a 35A BMS. Figure about 40AH capacity, should provide for days of dashcam parking time (good for airport parking for example).

As said here, DC-DC converters tend to be pricey. But using a solar charge controller seems to be the way to go. Not much $$, 10A/20A/30A versions available a-plenty, just pick one that will charge at an appropriate rate, within reason. I think a 20A one would suffice - not too much hit on the alternator. For the fun car I had already replaced the oem 100A alternator with a 200A one (and upgraded feed wire), so capacity to spare.

So, question. Any charge controllers come recommended ? Or, any to stay away from ? Something like this in the 20A or 30A version looks reasonable

https://www.amazon.com/ExpertPower-Controller-Auto-Select-Intelligent-Regulator/dp/B08ZK1NMS7

I don't see where it tells what the minimum input Voltage is. But, it's more than likely higher than your alternator output.

 

[MisterSandals]

The pack is a 4S/5P configuration of Headway 38120 8AH batteries, with a 35A BMS.

I have a similar situation - a 4S LiFePo battery

A lithium iron polonium 4S5P config should need 5 BMSs. Or do you have 5 cells in parallel and 4 of those in series (5P4S)?

 

[Halfwalker]

Heh sorry, yeah, 5 cells in parallel, 4 groups in series.

That solar charge controller sample I provided above won't work - needs at least 16V input for charging 4S LiFePo4, so no good with a 13.xV from the car system. BUT, this looks like it would do the trick ...

https://www.amazon.com/dp/B08X6HH5GW/?coliid=I1IESCYOK6BIQI

Maybe have one of these in the car for longer parking periods

https://www.amazon.com/dp/B0863QDYC9/?coliid=I1JZEFXPUT33AQ

Unroll that on the dash and plug into the solar input. Only a couple of amps, but would help the runtime somewhat.

 

[MisterSandals]

Unroll that on the dash and plug into the solar input.

Usually SCCs require a few volts over battery charging voltage in order to initiate charging. With the solar panel output at 14.4V and the LiFePO4 charge voltage at 14.4V, it is probably not enough.

Also, it looks like that solar panel has a built in charger so you would not connect a charger to a charge. I think the panel plugs into a cigarette lighter plug (one that is live when the car is off). Holding a LiFePO4 battery at 14.4V is not a good thing but that depends on how much sun you have and the battery.

 

[Halfwalker]

That solar panel provides a 14.4V output - that would go into the Kanruis charger as the solar input, alongside the main car battery input. But unfortunately I think the Kanruis solar input requires at least 16V - the car battery input only needs 13V. Shouldn't be too hard to snag the output of the panel directly which is likely around 18V or so.

From the description, the Kanruis will prioritize the car battery as a charge source, but switch to the solar input as required. Since the car battery would be disconnected via relay when the car is off, the only source then would be the panel.

 

[Halfwalker]

Thought I would update here - I got the Kanruis charger and the solar panel. The panel turned out to be easy to get into, and the solar input wires were right there with nice large pads on the pcb. Easy to solder a 3.5mm pigtail leading out through a small hole drilled in the side of the enclosure. The panel provides a perfect 25V.

I built two battery packs, one for my daily and one for the wife's Kia Sportage (the 1st guinea pig). 20x 38120 Headway cells at 8AH each in 5P4S, so a total of 40AH.

The whole setup works perfectly. While parked the solar panel is plugged in and trickle-charges up 30W - easy to unplug and stow. While driving it pulls around 20A from the alternator to charge. Capacity is more than enough - initial test had 4 days no problem, cells were only down to about 3.1-3.2V.

Final result for battery

I actually wrapped that in a thin layer of closed-cell foam and another layer of blue heat-shrink. It's strapped down pretty well in the back of the Kia under the rear floorboard, but just in case, the extra padding will help. Charge rate is only 20A max, so the batteries don't heat up.

I figure I'll attach a 5A balancer every couple of months or so just to ensure the cells stay level. Picked this one up, seems to work really well. WAY faster than the typical 100mA ones

https://www.amazon.com/dp/B09QKWH6RL