Thursday, Mar 29, 2012 at 10:58
Hi Leigh,
interesting point you're bringing up.
Just the other day I had a customer with a late model Prado with this diode boosted alternator.
He was a bit cautious about the unregulated high charging current this increased voltage would cause through a discharged auxiliary battery.
I told him this may or may not be a problem, depending on the size/length of wires involved.
Since the internal resistance of a discharged battery is 4 or 5 times higher, battery heating may become a problem under high charging current conditions.
And since higher battery temperatures decrease the gassing threshold, it'll start gassing at a lower charging voltage.
At 25 degrees, gassing starts at about 14.2V, but this threshold could be as low as 13.6V @ about 50 degrees.
Because the alternator happily pushes 50 amps and more through the battery at this voltage level, and electrolyte disappears at the rate of about 1 gram/Ah while in the gassing region, there's going to be some loss of electrolyte.
Counter measures are thinner and longer wires which restrict the max charging current somewhat.
And you only have to reduce the max current by 30% to achieve 50% less heating (I^2*R), which could be all it takes to keep outside the gassing region.
Aim for at least 20 milliOhm of wire resistance: for #6 wire, that's a length of 14m, for a distance of 7m between aux battery and alternator.
Going heavy and short in wire size in connection with a diode boosted alternator is a good recipe for a dead battery half way down the track.
Another trick to overcome the high charging current problem is, to simply install a second aux battery in parallel.
This helps in two ways: the high alternator current is split between the two batteries, and secondly the depth of discharge will be lower, thus the batteries have lower internal resistance which makes for less heating.
cheers, Peter
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