AUX Battery/Alternator charging design (beating my head against the wall)

Thread Starter


Joined Jul 6, 2016
Hello all,

Around a year ago I was working on this project that got put on the backburner for a while. Well now its back and I'm stumped on what seems to be a simple problem that I just can't get right.

The basics of the system is this:

Two boxes. First box is the aux battery box that ties into the car. Second box is a large DC load of various computers/sensors/cameras ect. The second box takes in a variable DC load from the battery box and does the regulation to various values, 5V, 12V, 19V and 48V specifically. The second box works great, no issues.

My main headache with the first box is trying to tie it seamlessly into the vehicles battery/alternator. What I want to have happen is when the vehicle is running, the vehicle power powers the second box, and when the vehicle is not running an auxillary battery in the first box powers the second box. Additionally, an AC powered battery charger is included so the auxillary battery can be recharged with wall power if needed. I drew up a rough sketch of what I think is relevant to the problem at hand.

What happens is with the car off, I turn on the system. Everything boots up, my battery monitor shows about a -7 amp discharge rate. If I plug AC power in my battery charger kicks in. With the system off I will charge fine. If I try to both have the system running and charging at the same time with AC power I get mixed results, sometimes a positive charge rate sometimes a negative discharge rate. The charger part number is JAC1212. Having the system run with the charger on is not a mandatory requirement, so while I would love to have that part work flawlessly its not that important.

However, with AC power removed and I turn the car on, which closes the solid state relay (we control that via software), and connects me to the car'
s battery, I start to see maybe a -4 to -5 negative discharge rate. This is where what I want to happen is I see either a positive charge rate or right around zero, so that the system is running and my battery is topped off to full/near full charge at all times, so in the event of the vehicle turning off, the system can remain running for an hour or two on battery.

Instead, my monitor always shows a negative discharge rate. I'm not sure if its really letting the battery go dead, if my monitor is wired wrong/showing wrong stuff, if my idea if fundamentally flawed?

Anyone have any ideas what I am missing in the equation/what will fix this? Some info that may be appropriate:

Battery Monitor is Victron BMV-700
Battery is US Battery AGM 35Amp U1
Charger is JAC1212
DC Load is around 7 amps, being regulated to 5V, 12V, 19V and 48V.