question

Bertus avatar image

using an MPPT as battery charger using main car battery as input

Hi,


In my camper I have 3 changing/power options in parallel for my auxiliary batteries:

1)Quattro 12V based system

2)MPPT 75/15 solar charger

3)24V to 12 V converter connected to the main car batteries while driving.


The 24V to 12V converter does not charge the auxiliary batteries well; it does not regulate the output voltage properly. I am able to operate refrigerator, lights etc. by using this system in case the auxiliary battery is empty.


To be able to charge the auxiliary battery while driving I came up with the idea to connect the 24V main battery to the solar input of the MPPT75/15 and disconnecting (and float) the solar panel (relay). In this case the car generator would act like a ‘solar panel’, generating about 24-28V ‘sun’ voltage.

The MPPT does manage the output voltage well for charging of the batteries and I should be able to charge the auxiliary batteries properly.


Would this be a good option? And a smart choice to do? Or can I expect issues?


Thanks for your help, B

MPPT - Solar Charge Controllerbattery charging
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1 Answer
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WKirby answered ·

In theory this would probably work.
However, the MPPT input is not isolated from its output and is designed to be supplied from a floating power source, the PV array.
In your case, in a vehicle, the 24V input to the MPPT would be grounded, as would the 12V output. You will probably end up with a ground loop of some sort.
As long as you are aware that you would be using the MPPT outside of it's intended use and to expect it to either not work or emit some smoke then you might get away with it.

I have tried this before, but in my case the two batteries were not sharing a common ground.

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Hey WKirby,


I'm interested in doing something similar (except instead of using 24v DC as input source, using 200+vDC (main traction pack in a different EV), with a 250/x controller ), for a DC coupled system. Input and output won't share a common ground.


What input voltage were you using? And do you know if it's possible to tie 2 of the MPPT input strings together? The voltages will always be the same on the 2 strings.


Thanks in advance.