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(answered) Battery-to-battery charger with higher amperage than source?

mgsloan

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Hey all, I realize this is a super basic question, but I imagine someone here knows the answer quickly, and I want to make sure before making purchases.

Would it work to connect the 15amp output of a lithium inergy kodiak portable battery to the input of a dc-to-dc charger that is 30 amps?

I'm not concerned about the lithium battery, more the gauge of the female cigarrette lighter ports / wires to them. Maybe I could use an inline current limiter or something?

More context: I want to charge my van's 200ah agm house battery off a portable lithium battery I already happen to have a 90ah inergy kodiak. I am looking at buying a DC-to-DC charger such as a Victron Orion, and considering going with a 30amp model instead of say the 9amp one, in case I want to upgrade the house battery to lithium in the future, and use it for alternator charging.

Even more context answering "why": my motivation for doing battery-to-battery charging is that solar alone is no longer sufficient for keeping my house battery charged, and it is necessary in the winter to keep the webasto gasoline heater running, to avoid the water system freezing. In the meantime I have the van trickle charging at 2amps from grid power, and this seems to have done the trick. However, I want to have more flexibility in parking locations.
 
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amperage = amps.

capacity = amp hours.

Assuming you mean amperage, no. You can't have less amps going in than you have coming out unless it's stepping down from a higher voltage, i.e., 24 to 12V.

If you're talking about a lower capacity battery as the source to charge a higher capacity battery, sure.
 
Right, I'm not anticipating 30amps of charging rate. My question is whether the dc-to-dc charger will try to pull too many amps from the car outlet on my portable lithium pack, which is rated for 15amps. Lets say the wires inside the kodiak are too thin to handle 30amps. I'm concerned they might heat up excessively.

I haven't dug into the options for current limiters very much, but I know such things exist. Could that be a solution to using a higher amp dc-to-dc charger with the kodiak?
 
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Right, I'm not anticipating 30amps of charging rate. My question is whether the dc-to-dc charger will try to pull too many amps from the car outlet on my portable lithium pack, which is rated for 15amps. Lets say the wires inside the kodiak are too thin to handle 30amps. I'm concerned they might heat up excessively.

Yes. It will pull whatever it needs dictated by the output. It assumes you've connected it to a suitable power supply. If you want to limit the input, you need to limit the output.

With a 15A limit, I'd specify a 10A or lower output limit.
 
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A 30A Victron Orion will try to pull around 35 amps from the input source in order to charge the battery connected to its output at 30 amps.
Clearly the source cannot meet this demand and the Orion will shut down, with strong possibility the supplying unit will be damaged.

Mike
 
A 30A Victron Orion will try to pull around 35 amps from the input source in order to charge the battery connected to its output at 30 amps.
Clearly the source cannot meet this demand and the Orion will shut down, with strong possibility the supplying unit will be damaged.

Mike
 
Awesome, thanks so much sunshine_eggo and mikefitz!

So I guess the 9amp one would be perfect. Bit of a bummer because I like the idea of having a more powerful charger for future alternator use if I upgrade to lithium (instead of current battery isolator).
 
Ah, new plan! I'll get a renogy 20A dc to dc charger, then raise the LC current limiting input of the dc-dc converter, which should switch the charger to 10A mode. The input will be protected by fuse. If this doesn't work, I have the option of using the other cigarrette lighter output of the kodiak in parallel with the first, for a total of 30A.

Specific components, if someone is curious / looking at this in the future (will try to remember to write back here whether it worked):

* Amazon: Battery Temperature Sensor for 12V 20A/40A/60A DC to DC on-board Battery Charger : Automotive
* Amazon: Renogy 12V 20A DC to DC On-Board Battery Charger for Flooded, Gel, AGM, and Lithium, Using Multi-Stage Charging in RVs, Commercial Vehicles, Boats, Yachts, 20A : Automotive
* Amazon: Battery Cables – LotFancy 8 AWG 24 Inches Power Inverter Cables for Car Marine Solar ATV Lawn Mower Motorcycle, 3/8" Lugs (Positive and Negative) : Automotive
* Amazon: SCCKE Heavy-Duty 14AWG 15A Male Plug Cigarette Lighter Adapter Power Supply Cord with 2ft / 0.6m 14 AWG Cable Wire for Car Inverter Air Pump Electric Cup and DIY : Automotive

Total: $135
 
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Wouldn't it be simpler to just power the diesel heater directly from the solar generator? You would effectively have more power available that way, as the DC-DC charging of your AGM is going to be grossly inefficient. For every aH you pulled from the solar generator you're AGM is going to accept only some fraction of it. That would be power better spent running the heater.

Personally, I would rather have the DC-DC charger wired up to my alternator instead.
 
True! I'm not actually using the inergy kodiak for solar, though I did intend it for that purpose when I got it a few years ago. Instead a renogy rover 60A + 480W of panels connected to the 200ah AGM battery. It has a battery isolator for alternator charging (the DC-to-DC is only required for lithium house battery).

Another use of the DC to DC charger is low charge rate grid power. I know it's terribly inefficient, but I can connect a 120v AC -> 12V DC converter to the DC to DC charger.

Unfortunately I don't have any good places to park the van that gets direct sun during the winter, and recently the solar has not kept up with the heater's demands. Also unfortunately, I think the battery has already been overdischarged a few times.. So that may be part of why the system is not keeping up with the demands (and part of why I am considering lithium)

There may be something wrong with the heater causing it to consume more electricity than it should, as I've noticed it turning on even after I've heated the van to 80F by running the engine and van air system, which seems odd.
 
On further thought, you do bring up a very good point. The heater draws less than 15A so I could run it directly off the kodiak's cigarette lighter. To be safe, I'll try running it off of the two cigarette lighters in parallel.

I think I went down this charging rabbit hole due to wanting to also ensure I don't overdischarge the AGM battery any further, but I could also do that by just disconnecting the loads.
 
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Update:

* Running the van's electrical off of two of the kodiak's 15A cigarrette lighter plugs worked pretty well. However, after a day or so the webasto (air top evo 40) was still getting upset about low voltage, even when the inergy kodiak was on an 8A charger. Unfortunately that heater is really unresilient to faults. As soon as something goes slightly wrong it gives up and manual intervention is required to restart.

* Gave up on keeping the van on battery during super cold / overcast days, and instead plugging it into shore power via a Victron Blue Smart IP65 15A charger. Opted for the IP65 model instead of IP22 since it seems more versatile and I don't really need 20A, and I liked that this comes with cables for using it as a charger for a normal car battery.

Probably could have gotten a AC-to-DC and used it with a DC-to-DC charger, but that'd be inefficient. I also figure that if I upgrade to lithium in the future and get the DC-to-DC, then I'll want to use the DC-to-DC exclusively for the alternator.

I've searched around a bit and haven't seen anything say whether it'd be ok to wire the output of an AC-to-DC converter in parallel with the alternator. It sounds dangerous. If someone knows of a nicely packaged DC source switchover (I'm thinking pretty much just a heavy duty DPDT relay connected to lugs), then I'd consider going that route instead and return the victron charger.
 
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