Vehicle DC-DC charging

Not being an expert, but a reasonably competent electronics person, I wanted to figure out a way to limit current when charging a LiFePO4 battery from a running vehicle. Sure, you can buy appropriate devices, but I like to build things. I went back to the old "Dim Bulb Current Limiter". This design puts a bulb in series with the load, and it limits the current to what the bulb would carry if it was directly across the supply. In my case, I had several 1156 automotive bulbs hanging around. These bulbs draw about 2 Amps max, so 5 bulbs paralleled would draw a max of about 10 Amps. This bulb load, and series with the car charging of a LiFePO4 battery, would limit the charging to about 10 amps. I didn't have an appropriate diode hanging around, so this is currently a bi-directional device, and would only charge the LiFePO4 if the vehicle is running, and should be unplugged before shutting the vehicle off. In any case, the more current flowing, the brighter the bulb glow.
Sure, it's not very elegant, and not very efficient, but sometimes old things can be useful.

Let us know how it performs, and post a build of it.
I’d put a relay in on the ignition or accessory switch lead though…

I'll play with it a bit. This is a portable plug in to the lighter outlet, I don't think that I need a relay (an inline switch might be good), but a diode to prevent back flow from the LI might be good. I have to get it set up on my car, with a partially discharged Li, and see what the alternator V is. It should be around 14v, which isn't a bad voltage to charge a 12v LiFePO4. A diode would drop the V by about 1/2V, so you don't want to go too low..

Won't there be a voltage drop across the lamps, and won't that bring things down to far to charge the battery?

Doug Wall said:

I wanted to figure out a way to limit current when charging a LiFePO4 battery from a running vehicle.

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Also see this thread with related concepts.

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Another approach is to get a 12v @ 6a or 12v @ 8a battery charger - e.g. limit what the charger can do by design (and it's cheaper). These are typically 110v -> 12v @ Xa but can powered by a 12v -> 110v inverter. Not that expensive and has the advantage of simplicity .

TacomaJoe said:

Won't there be a voltage drop across the lamps, and won't that bring things down to far to charge the battery?

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That's the beauty of this type of device. When the voltage difference is high, say trying to charge a big LiFePO4 that's at say 12.5V, and your alternator is running at 14V, you could potentially dump a hundred amps into the battery, and burn out the alternator. This device will limit that to 10 Amps (The bulbs don't care about voltage, only current). When the battery is almost fully charged, and there is a low current, the non-linear resistance of the filament drops to a very low value and there is very little voltage drop. The resistance of a cold bulb is only about 1/10th that of a hot bulb. Look up dim bulb current limiters. They are used more for protecting low wattage appliances (line voltage) from shorts during testing.

OffGridInTheCity said:

Another approach is to get a 12v @ 6a or 12v @ 8a battery charger - e.g. limit what the charger can do by design (and it's cheaper). These are typically 110v -> 12v @ Xa but can powered by a 12v -> 110v inverter. Not that expensive and has the advantage of simplicity .

Click to expand...

So you get a battery charger, and an inverter, to replace 5 light bulbs and sockets. For that price, you can just go out and buy a Renogy DC-DC charger

Doug Wall said:

So you get a battery charger, and an inverter, to replace 5 light bulbs and circuits. For that price, you can just go out and buy a Renogy DC-DC charger

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Agreed - but both the inverter and the charger are useful for other purposes on their own. Its just an option

Doug Wall said:

I didn't have an appropriate diode hanging around, so this is currently a bi-directional device, and would only charge the LiFePO4 if the vehicle is running, and should be unplugged before shutting the vehicle off.

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The bulbs would also limit the watts in reverse when starting the battery. Maybe no need to isolate??....the resting voltage of the two batteries probably the determining factor.

Couldn't you use a cc/cv boost converter since you need to boost voltage to 14.6v?

I have a LiFePO4 12V battery in the back, I forget exactly why, charged from my 24V system with a cheap (communist) Chinese buck converter (with CC "limiting"). To keep my starter battery topped off, I connected it using a cheap (communist) Chinese SAE cable, or three (one end, extension, other end), (14AWG, if I recall). Somewhere in the line, there is a 15A fuse, or two, which don't blow very often, because the voltage drop in the cable pretty much keeps things under control.

NVS said:

Couldn't you use a cc/cv boost converter since you need to boost voltage to 14.6v?

https://smile.amazon.com/dp/B07N6BJWXH/ref=cm_sw_r_apan_i_8PEZT1RV3GAA2JMY0AXY?_encoding=UTF8&psc=1

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I thought about that, and did a quick search. It appears that on boost/buck converters, at least some, want at least a 3 volt difference between input and output. Boosting up to 18 volts with one converter, then back down to 14 volts with a second converter might be a possibility.

The lithium battery will settle, when charged, to around 13.4 volts. This is not high enough to push significant current back to the starter battery. A low volt drop power diode may help with I would try without for a start.
When the lithium battery is discharged it will have to drop below 30% SOC before there is any flow out of the starter battery.

Mike