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ColinS avatar image

How to prevent alternator damage when VE.Bus BMS cuts the charge to LiFeP04 batteries?

I understand that under certain conditions (over temp/over voltage etc)the VE.BUS BMS will cut the charge from the alternator to protect the LiFeP04 batteries. However, effectively cutting the alternator off when under way could potentially damage the alternator. I have seen one solution which was to also connect a small lead-acid battery (with a small continuous load) added directly to the alternator output but this seems to be a bit of a mash-up! Is there a more elegant way of protecting the alternator from a sudden load disconnect?

BMSLithium battery Smartalternator
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3 Answers
Mub avatar image

I have a Balmar MC-614 external regulator controller for the house battery alternator. After talking to Balmar and others the solution I implemented with is to add a solid state normally closed relay to the MC-614's ignition line.

I took the BMS alarm signal and used this to drive the relay. This means that the alternator regulator does an emergency shutdown of the alternator rather than allowing unknown behavior.

I tested this solution several times at full load running ~180A in to the house batteries. The worst spike we saw was about 20 volts on the 12V bus. Monitoring the bus with a Keysight 350MHz scope.

There seemed to be very little delay between the BMS triggering a disconnect and the alarm.

The BMS and relay are powered from the engine battery which is a SLA and has a small(er) alternator of it's own. I didn't want a house battery event causing a primary engine shutdown.

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Which BMS are you using? The VE.Bus BMS? I just received one of those and it came without the pre-alarm output. I'm trying to work through this issue with the vendor.


thanks,

bill

Charlie Johnson avatar image

Sorry, I was very, very unclear! I assumed that the OP was using an external, programmable, alternator to match the required LFP charging profile, in which case a relay in the B+ to the external regulator is the appropriate solution.

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Charlie Johnson avatar image

Turn the alternator off with a relay in its B+ conductor. Don't use the field wire.

And the lead acid battery hanging on the alternator output is not that big of a "mash-up" IMHO.

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Please note that not every alternator switches off when B+ is disconnected, most (?) automotive alternators keep producing once energized.