Dc-dc Charger and service battery as backup to start engine.

[Frasco09]

Hi everyone,
Looking to build a electrical system in my jeep with a lithium battery for services, fridge, inverter, ecc charged by a Dc-Dc charger.
I would like to have the chance of reversing the power flow should I need to use the battery to start the engine, or, for example, help with prolonged winch operation. Since it would be a rare situation, i was thinking of installing a switch to bypass the charger and link the service battery directly to start battery. This would bring alternator polarity on both the input and output of the charger. Would this damage it? Specifically looking at a Renogy dc-dc 50amp, which advertizes "protection for inverted polarity", guess it would do the job...?
Any suggestions?
Thanks in advance to anybody who might contribute to solving my doubts!

 

[clydeps]

It would probably do no harm, but it would be best to use a changeover switch (so that the charger was disconnected) to avoid the problem. The other issue is what do you expect to happen when the service battery is switched in parallel with the vehicle battery?

Are you intending using a big enough cable so the service battery can supply starting current? Lithium batteries of the type used as RV house batteries are not usually intended to supply the very high peak currents needed to start an engine, and in any case you would need the right cable and switch to handle the current without excessive voltage drop.

Running a winch is a more realistic objective which would not require as high a current capacity.

 

[Frasco09]

Using a chamgeover switch would still allow alternator output to reach the charger (on its output) via the lithium battery, right? This concerns me more than having equal tension on both sides, even though I have no idea how the circuits handle both situations. Was Just hoping not to have "Stuck errors" in the charger requiring time-consuming troubleshooting to reset.
Yeah i was mainly thinking of having them in parallel for Major loads like winching, or, viceversa, some Power thirsty tool on the inverter. Just to avoid having 3 batteries for a "once-in-a-lifetime" Need ?
I used to run a cable to the AGM battery with a manuale switch to isolate It, I remember running calculations on the cable for 150a worst case, its pretty beefy and should be up to the task.

 

[clydeps]

Using a chamgeover switch would still allow alternator output to reach the charger (on its output) via the lithium battery, right?

Only if you wired the changeover on the input to the charger. That also should do no harm because the charger has to be able to cope with having a battery on its output without any power on its input. It's the same reason you can wire multiple chargers (e.g. DC-DC, solar, mains) in parallel to the same battery - they have to be able to deal with the battery still being connected but the input supply disappearing.

Also the Renogy charger has a voltage or ignition sensitive disconnect relay on the alternator input so it will be disconnected when the alternator is not running, so there would be little benefit in switching the input if that's what you're using.

But I would wire a changeover switch with the common terminal to the house battery, and the switched terminals to the charger output and the main battery respectively.

The bottom line is that what you proposed should work, the changeover switch would just provide some extra isolation.

 

[clydeps]

Oh, and the protection against reversed polarity is not relevant here - that would only come into play if you swapped positive and ground.

 

[john61ct]

Blue Sea A/B switch

 

[Frasco09]

Only if you wired the changeover on the input to the charger. That also should do no harm because the charger has to be able to cope with having a battery on its output without any power on its input. It's the same reason you can wire multiple chargers (e.g. DC-DC, solar, mains) in parallel to the same battery - they have to be able to deal with the battery still being connected but the input supply disappearing.

Also the Renogy charger has a voltage or ignition sensitive disconnect relay on the alternator input so it will be disconnected when the alternator is not running, so there would be little benefit in switching the input if that's what you're using.

But I would wire a changeover switch with the common terminal to the house battery, and the switched terminals to the charger output and the main battery respectively.

The bottom line is that what you proposed should work, the changeover switch would just provide some extra isolation.

Oh, great, i was thinking of the switching being between charger input and house battery, with the main on the alternator. It makes sense as you're suggesting! Since i would have to out a manual switch, It might well be a changeover One!

Blue Sea A/B switch

Thanks! Didn't know this One! I have to look carefully into the specs, looks like It would get the job done, Just Need to be sure the automatic functions don't create any problems, doesn't look like at First glance.

 

[john61ct]

I meant the purely manual one, no automatic feature there.

Using one of their powerful latching ACRs with the jumpstart feature is another option, but that is the basis for a fundamental reworking of your overall design