T-Pain Is On A Boat: [Electric Boats] Re: Batteries wired in parallel (and series...)

Sunday, May 1, 2016

[Electric Boats] Re: Batteries wired in parallel (and series...)

I have always been the "hands on" type with system monitoring. For my 48V 5Kw system, 4-12V AGM in series, for a small boat, I use voltage and temperature monitoring with alarms per battery. I recharge batteries individually with a temperature compensated quality charger. And I pay attention to my system. So my protection system is "me".

I have used series-parallel battery stacks, but for a house bank in a boat, and for that I never had batteries out of balance or over temperature even when charging with the 200A alternator on the ICE. Of course the batteries were all same size/age and recharged routinely after 30% discharge. I also replaced them when the annual capacity tests showed significant loss of capacity (typically 8-9 years).

But I can see some advantages for a series-parallel system for lead acid batteries where battery physical size is an issue. For that application one would want some protection circuitry in the parallel pairs as well as on the string. Miles pretty well covered it regarding fusing and diode isolation. Miles' concern with mosfets was failures, but with enough voltage and current overhead on the device with adequate heat sink that would not be expected as there are some very high current mosfets at reasonable voltage levels.

I have used back to back mosfets (drains connected together) in power supply circuits for protection and switching. In power supply circuits we cannot isolate the negative side so easily, so the mosfet pair is on the high side (+) . But in a series-parallel string you can, for control purposes, treat each battery as a separate 'system' only needing opto-isolated control signals from the system control end of things. To avoid high side drivers just put N-channel mosfets in the negative lead of the battery (isolating the battery on either pole works ! ) then switch the gates and power the logic with the individual battery's voltage. All very doable at 12V, and the ability to isolate any battery from the string on failure. Add a temperature sensor per battery, and monitor the voltage drop across the mosfet pair for overcurrent protection.

Not as elegant as the commercial ($$$$$) solution, but effective battery failure protection. I don't think the circuitry described is beyond the technical capabilities of many of our list members.

There is also the fun/satisfaction of making it yourself!

John Acord

Flatwater Electronics
www.flatwaterfarm.com
"Neurosurgery for computer looms."

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Posted by: John Acord <jcacord@gmail.com>

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