question

jo-hnno avatar image
jo-hnno asked

Disconnect switch between mppt and battery

I have a small sail boat with a 70ah lithium battery which is connected to a switch panel and has a disconnect switch adjacent to the battery which is on the active wire. All that is connected to the battery is a chartplotter, radio and a few led lights. There is no engine, no alternator, no inverter etc - very simple. I use the disconnect switch to isolate the battery when I leave the boat.
I am now in the process of adding a 70w flexible solar panel, with a Victron mppt 75/10 smartcontroller. Normally the solar panel will top up the battery while I’m away. Normally I will disconnect the panel when I come to the boat to get it out of the way. When the sail is over I will reconnect the pv and go home. But sometimes, like today, I want to disconnect the panels and leave the boat for a week or more. I am worried about leaving the controller connected to the battery and fear it will run it down. It is impractical to disconnect the controller at either the controller or the battery end. I am thinking I would like to put a switch in the negative between controller and battery. Is this okay to do and/or is there a smarter way to do it? I figure this must be a common issue and potential set-up but I can’t fi d anything on it here. Thanks very much for your help.

switch
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2 Answers
Stefanie avatar image
Stefanie answered ·

Hi @jo_hnno,

the BMS of your battery should take care of the MPPT and in the event of high cell voltage or low temperature should disconnect the MPPT for safety reasons.

For the switch, yes that's ok. However, without the BMS controlling the MPPT, this again comes with a high risk in case the MPPT is topping up your battery while you're not there.

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jo-hnno avatar image jo-hnno commented ·
Thank you Stephanie. Can I confirm that you are saying there is no risk of the controller running down the battery because the BMS of the battery will prevent that? And that I do not need to either disconnect the MPPT nor install a switch. I can just leave the controller connected to the battery for one or two weeks with no drainage issues for the battery? Thanks.
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jo-hnno avatar image jo-hnno jo-hnno commented ·
Sorry, I should have said ' okay to leave it for a week or two with the controller connected to the battery but without the panel connected to the controller'.
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Stefanie avatar image Stefanie ♦♦ jo-hnno commented ·
The self consumption of the 75/10 is 10-20mA. That's roughly 0.5A per day. So it's no problem to leave the charge controller for a week or two provided that the battery was fully charged before you disconnect the solar panel.
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Stefanie avatar image Stefanie ♦♦ jo-hnno commented ·
No, I'm not saying that. This is only valid if your BMS is properly installed and has a relay that will disconnect the solar charger if something goes wrong. I don't know what your system looks like and how it was designed.
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jo-hnno avatar image
jo-hnno answered ·

Thanks again. As you can tell, this is all a mystery to me. I appreciate your help.

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