No big deal - a standard problem for boats, caravans and rvs.
You want a split charging system.
You can link the two batteries through contactors and power rectifiers
to a common output power rail. Then have a voltage comparator with a lot
of hysteresis across each battery. The one across the deep discharge
battery will trip its contactor when the battery is near exhaustion. The
one across the starting battery will trip much earlier, so that there is
still power in the battery to enable a restart.
You need a lot of hysteresis because the battery voltage will increase
when the load is removed with the contactor opening. Setting the trip
reset point to a voltage only reached when charging is needed.
> I know motor control
> and dabble in electronics. Please show me
If you know motor control, you know about contactors and power
rectifiers. You also know about designing protection circuitry and
preventing fault propagation.
If you dabble in electronics, then you can design and build a voltage
comparator and a driver to power the contactor. Designing hysteresis
into a comparator is elementary electronics.
If you have a specific query, I may be able to help.
This is dc, so an SCR as a switch is a bit problematic. Once switched
on, they can be quite difficult to switch back off again. It can be
done, but cheaper power rectifiers are all that is needed in this
All power rectifiers are diodes - not all diodes are power rectifiers.
You want diodes able to carry the current and with low forward voltage
drop - power rectifiers are the type of diode designed to be used in
such roles. They often need to have the chip bonded onto a big chunk of
metal, in order to be able to transfer waste heat into a heatsink.
Typically, you take a wire from the battery to a fusible link - or
resettable current trip - anything to protect the battery from a fault.
I usually use a battery connector that includes a manual
From the protection device, the wire goes on to the contactor contact.
From the other contactor contact, a wire goes to one of the power
rectifiers. The other power rectifier, with a wire from the other
battery, via its own protection device and contactor, can usually be
bolted to the same heatsink. This heatsink is then the common output
Obviously the heatsink has both batteries connected to it and so it is
"live". There would be a big bang if this heatsink gets connected to the
battery return. But, hopefully, the protection device(s) will limit the
damage. You can get "semiconductor" fuses that should protect the power
rectifiers - if there is going to be a possibility that this could
happen. They are specially designed to blow faster than the rectifier
diode, under fault conditions.
Alright, yeah, uhmn, I'm seeing the light. I have been looking at
comparator, any particular kind quad, dual, are there power specs and
a voltage range say 12 to 48 if I make a bigger inverter setup
and as always thanx.................
You are going to need at least 2, one to control each contactor. You can
either use 2, with feedback to give the hysteresis needed, or use 4 and
set two of them to the trip out voltage levels and two for the reset
You can power the electronics with a small regulator circuit - designed
correctly the latter will be able to cope with a wide range of supplies.
The electronics will use so little power that you can use a simple
potential divider to drop the supply voltage to the comparator input. If
you change the number of batteries, you just have to change the ratio of
the values of the resistors in the potential divider, to compensate for
the extra input voltage.
I would suggest experimenting on a workbench with a small, variable
voltage supply or even a handfull of AA batteries as the power source -
full size car/rv batteries can be a bit unforgiving of mistakes.
If all that looks a bit too much to take on, then the sort of thing
proposed by Binger could be used. Rather than the one he suggested, I
would suggest a couple of:
These have all the electronics you need - and you can set them to
operate at different points, so the "charge" battery trip point can be
set higher than the deep discharge battery.
Connect each of these to its own battery, set the trip points to what
you want, combine the outputs with a pair of power rectifiers, as
previously proposed, add battery protection devices and you are in
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