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Low Current DC-DC Charger for Small LiFePO4 Batteries

Just An RVer

No Step On Snek
Joined
Apr 21, 2021
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34
Can anyone recommend some low current DC-DC chargers for 16-20 ah batteries? I'd like to limit the current to something like 5-8 amps. Everything I've found searching the forums looks like it's intended to charge a large battery or battery bank in an RV or boat. Can any of the "smart" DC-DC chargers be programmed to limit the charge current that low? The lowest current model I've found so far is the Renogy 20a, which it seems can be set to half-current at 10a.

Our work vehicles have an always-on DC load (a telemetry and messaging system with some cellular and radio gear) that I'd like to separate from the starter battery, both for noise suppression and to keep the starter battery from being drained if parked for several days.

In the past, when tried to charged my own Miady batteries directly from the alternator voltage, it drew more current than I was comfortable using to charge that little battery, so I know I need something to limit the current. If I used a relay activated by the ignition, would a simple voltage regulator like this work:

Probably just wishful thinking, but it sure would be convenient if it turned on and off based on voltage, like the Victron units, so I don't have to run a separate ignition sense wire. I'm sure someone makes a stand-alone voltage-sensing relay that does this, but I haven't found one yet.
 
One last thing: No one's life depends on this gear. A budget brand like Renogy isn't out of the question if the product has somewhat of a track record. While I'd certainly prefer a ready-made solution, I also realize that there just might not be a commercial product like this. If I have to cobble together a solution from unbranded Chinese components, I can test it on my personal vehicle for a few months before I try using it for my business. The exterior appearance is still fairly important, or I'd just slap a small solar system in each of them.
 
Probably just wishful thinking, but it sure would be convenient if it turned on and off based on voltage, like the Victron units, so I don't have to run a separate ignition sense wire.
This works really well. I've installed a 30a Victron Orion. They make an 18a version that is harder to locate in case you didn't see it. I do not think they are configurable for charging amps unfortunately. They might have added this feature but the one i installed drove away (and happily charging).
 
This works really well. I've installed a 30a Victron Orion. They make an 18a version that is harder to locate in case you didn't see it. I do not think they are configurable for charging amps unfortunately. They might have added this feature but the one i installed drove away (and happily charging).
@MisterSandals thanks for mentioning the 18a Orion. I've seen that one online, but I think even that's more current than I comfortable with for a 16 or 20 ah battery. I know there used to be inexpensive 32 or 36 ah LiFePO4 batteries on Amazon, but I don't see any, now. Maybe I can find something that could handle 18a and still fit.

I guess I should join the Victron forums and ask there about limiting the charge current before I give up on it.

heres a dcdc charger we sell.it will work for 12 or 24 volt system.5 amp withdraw til supply battery hits 12.2
@gideonzook I don't see anything about that charger online, so I'll follow-up with you via PM. I thought 12.2 volts was pretty weak for a vehicle's starting battery, but that does seem to be the low-voltage cutoff on a lot of consumer devices.

That lead me to some low-voltage disconnect modules on Amazon that might be able to automatically turn a charger on and off. It's unclear how high the low-voltage point can be set or if the difference between the alternator's voltage and a fully-charged starting battery at rest is wide enough for them to be used. They're pretty inexpensive, so it wouldn't be the end of the world if it didn't work
 
Take a look at RC type chargers. They offer AC and DC to DC charging with adjustable amps etc.
 
Take a look at RC type chargers. They offer AC and DC to DC charging with adjustable amps etc.
@grizzzman Thanks so much! I'm always impressed by the breadth of knowledge here.

I'm not familiar at all with RC battery chargers, but at first glance, they look promising. Judging from a quick skim of Amazon reviews, the overall quality and voltage/current accuracy are all over the place. If I go this route, it will take some time to weed out the garbage, but I guess that's to be expected until you've got some experience with a particular product.

Surely one of these options will work. If I can automatically switch it on and off without a separate ignition sense cable, that will be icing on the cake.

Thanks again, everyone!
 
@grizzzman Thanks so much! I'm always impressed by the breadth of knowledge here.

I'm not familiar at all with RC battery chargers, but at first glance, they look promising. Judging from a quick skim of Amazon reviews, the overall quality and voltage/current accuracy are all over the place. If I go this route, it will take some time to weed out the garbage, but I guess that's to be expected until you've got some experience with a particular product.

Surely one of these options will work. If I can automatically switch it on and off without a separate ignition sense cable, that will be icing on the cake.

Thanks again, everyone!
I suggest the ISDT brand. https://www.amazon.com/ISDT-Touchsc...jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==
as an example.
 
Do you happen to know if that one will start charging when power is applied, with no user interaction?
Those do not. And once they reach target voltage, they stop and need to be restarted manually.

I have one of these from house batteries (solar charged continuously) to charge the sprinter engine battery. Its been installed and working for over 10 years. Got 9.5 years from engine battery keeping it maintained with just this trickle charger. There are a few different models with different applications. I do not think there is a mechanism to prevent over discharge of source battery.

 
I like the manual restart . Only use it when I need it. What is the target voltage?
 
Those do not. And once they reach target voltage, they stop and need to be restarted manually.
Well, that's a bummer. Charging on power-up would be required for this use case.
I like the manual restart . Only use it when I need it. What is the target voltage?
The idea is to power a constant load from a fairly small auxiliary battery, and charge whenever the engine is running. Manual activation of the charger just isn't practical.

I'm tempted to try a 12-to-24 volt step-up converter going into a solar charge controller. It won't be efficient, but I would have full control over the charging parameters. I just worry about RFI from the converter.
 
I was able to find a DC-DC boost converter that says it can output constant current (0.8‑22a) and constant voltage (12‑97v) with an adjustable low-voltage disconnect (10-50v), so I think I'm going to order one for testing. I don't trust the 30a "rating" enough to put any serious current through it, but maybe if I'm only asking it to boost from around 13 volts up to 14.2 volts at under 10 amps, it won't let the smoke out. At least they thought enough to use some silastic to stabilize the capacitors.


At one time, I had some aluminum project boxes with a heatsink on one side. They looked like tiny car stereo amplifiers. If this device works on the bench, I'd want it fully enclosed before I mounted it in my car.
 
You may be able to use a Tecmate-Optimate TM-V500 V3 dc-dc charger. 2.7A max charge.

Mostly designed to charge one battery from another, whilst protecting the "donor" battery from discharging too far if left on by accident. These modern versions say they support LFP.

I'm a fan of these two-terminal chargers for various reasons, but I have never tried one in your specific application, especially hands-off. I normally cut off the SAE connectors and use Andersons on all my stuff too.
 
I find myself in the same boat as the OP ! Small engine lead acid batteries never last more than a few years. A 12V LiFePO4 battery pack would last much longer.

Most small engine charging systems have little or NO regulation so the voltage can be 10V-18V. Clearly some regulation/charging solution needs to be in front that lithium battery
 
Adjustable bench power supply with a timer switch.

I charge anything from individual 18650 cells to 48v battery banks. Double check the voltage setting with a fluke multimeter.
 
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