24 Silicon chip Australia’s electronics magazine siliconchip.com.au
High Current
Solid State
12V Battery
Isolator
This device connects an auxiliary battery to the main vehicle battery/
alternator while the engine is running, charging that extra battery. But
it disconnects it once the engine shuts down, so that the vehicle battery
can’t accidentally go flat. It’s cheap and easy to build but also very
robust. It’s ideal for RVs, campers, offroad vehicles and boats.
by Bruce Boardman
(VK4MQ)I
have had ongoing problems with the battery systems
on my 4WD vehicles. My car is fitted with an auxiliary
12V battery system that I use to run a fridge, some ra-
dios, camping lighting etc.
I tried using a commercial battery isolator to connect it
to the main vehicle electrical system but found that this
had two major shortcomings.
Firstly, its case offered little protection from the elements,
and it occasionally filled with water – not good.
Secondly, it uses two open-frame style relays to con-
nect the batteries in parallel. The contacts in these relays
are nothing special and occasionally weld together, leav-
ing the batteries permanently connected. That can lead to
both batteries going flat. Also not good!
The idea of these isolators is to parallel the batteries
when the engine is running and remove this connection
when the engine is off.
So when you are camped overnight and you discharge
the auxiliary battery, you can still start the engine in the
morning.
It works by measuring the vehicle battery voltage, which
is usually below 13V with the engine off and around 13.5-
14.5V when the engine is running.
So when the voltage is high enough, it
determines that the alternator is charg-
ing the battery and connects the auxil-
iary battery.
When the voltage drops, it detects that the engine has
been stopped and breaks that connection.
Not being at all happy with the commercial units I tried,
I decided to design my own.
My design criteria were:- Low current drain from the main battery when the en-
gine is off. - Fully solid-state operation (no relays).
- A low forward voltage drop when switched on, minimis-
ing heating and power loss. - Must not interfere with radios (ie, no RFI/EMI).
- Must use commonly available parts.
- Must handle very high currents without damage (>100A).
- A completely waterproof and dustproof housing.
These made the first design decision easy: Mosfets are an
ideal solid-state switching device for large direct currents.
While P-channel Mosfets are easier to drive for high-
side switching, N-channel Mosfets offer lower losses at the
same price thanks to a vanishingly small ‘on-resistance’.
So I decided upon six Infineon IRFS7434TRL7PP Mos-
fets, which have an on-resistance of less than 1mΩ (0.001Ω)
and are each rated at 40V and 362A.
(I initially used similar IRFS3004-7PPBF devices in my
prototype, but these have now been dis-
continued).
The S7434TRL7PP Mosfets come in
a 7-pin D2PAK (TO-263) SMD package