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Split Charger

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New Member
I am trying to find a circuit for a charger that will charge 2 car batteries
from a 65 amp alternator. The difficulty is I need to be able to use one power the vehicle and one for accessories. The only ones I can find use a diode and you lose about 1v across the accessory battery. Is this the only way of achieving split charging or is there a way that could have full charge in both batteries while keeping them isolated
I have searched the internet with no luck can anybody help?


Active Member
Instead of using diode as isolator you can use other switching devices like power MOSFET. It provides minimum voltage drop.


New Member
I need to isolate them so that the accessories only draw from 1 battery
and leaves the other for starting. But I need full charge in the second battery as a backup for starting/accessories


Active Member
Hi jezzz,

you could modify the alternator itself.

Typically there are six diodes, in three pairs.

You could take a feed from one of the three,
leaving the remaining two pairs to do
their usual function,

The voltage control operated by the regulator
affects all three of the output windings so
the section separated for 'other' use would
still be regulated.

The construction of the diode pack varies from
maker to maker, generally it would not be that
difficult to take a separate feed from one of
the diode pairs.
You might have to examine the way its fitted
together to decide how to do it.

Such a feed to a separate battery would share
the ground connection, but would not share any
thing else.

best of luck with it,



Active Member
I think You need separate accus only when the motor no run. After starting can charge two accus parallel. With charge-control lamp signal possible control a relay for parallel switching.


Active Member
Yes, charging is a bit of a pig, cos putting a diode in
just for the other battery, means it wont get a proper
Thats why ive suggested splitting the existing diode pack
this means that the auxillary battery will get the same
treatment as the duty battery.
Except of course that the 'sensor' or regulator is on the
duty battery, so i suggest swapping that occasionally.

As to using both when starting the vehicle,
after some thought i would suggest just fitting two
starter solenoids, one for each battery, that way they
would only be joined for starting.
Both solenoid coils being wired together.

Best of luck with it,
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