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Off grid fridge project in west africa
#31
I have now been using the battery for a few weeks, everything works fine when charging, the cells all get well balanced (none has been in bypass mode yet, with the solar charger absorption voltage set at 3.4V, and bypass at 3.5V).
But when discharging, cell 9 & 8 starts to drop voltage much quicker than the others, the shunt was showing 9.5% left when cell 9 was at 2.8V:


Given that my inverter will shutdown at around 42V I think I need to add a circuit breaker and shunt trip to avoid the dangerous situation where cell 9 get drained to 0V, could switch polarity and create a short circuit. I understand the shunt can be controlled by the output A or B of the WM4.

Does anyone has a not too expensive shunt trip breaker to recomand?

I guess I should also replace cell 8&9 with new ones?

thanks for the help
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#32
(01-25-2019, 07:32 AM)babish Wrote: ok thanks, I will do.
what do you call "moderate" and "medium higher" current?
I usually have 2 loads, one ~ 50W and one 80/120W, so between ~ 1 and 4A, so I guess pretty small.
I will run only the 50W load first.
Also about charging should I still limit to low current for now, eg max 2A? Or how high can I go if I do not see overheating?

Hi Babish, sorry I missed this post.
Your cells are 40Ah capacity right?
So you should be able to discharge them comfortably at least a rate of 0.5C  where C = 40Ahrs so that's 20A discharge current. Some cells like your are rated 1C = 40A continuous discharge.
Note for longer cell life this is not recommended, a max continuous charge or discharge of about 0.25C is better ie 10A for yours.

With cells 8 & 9 you might try charging them to 3.45 or even 3.5V & see if they improve.
It does sound like their capacity is a bit low vs the others.
Running off solar, DIY & electronics fan :-)
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#33
(02-18-2019, 03:41 PM)babish Wrote: Does anyone has a not too expensive shunt trip breaker to recomand?

These, I doubt you'll find a better quality DC breaker at that price - they come with trip signalling contacts, you just need to add a trip (internally) with a coil voltage to suit your control system. Output A pulse from a WM4 works well.

https://www.ebay.co.uk/itm/Merlin-Gerin-...ect=mobile
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#34
(02-18-2019, 03:41 PM)babish Wrote: I guess I should also replace cell 8&9 with new ones?

thanks for the help

I don't use LFP so I'm not so sure about the operating ranges.  Those cells 8&9 are your lowest common denominator. So if you don't want to replace these two cells, then lower the expectancy of your other cells. Besides you don't want to discharge to 2.8V as a rule anyways. By doing a 100% depth of discharge only reduces your life expectancy. Going 80% will improve your life expectancy of your cells. I think 3.1V is a good lower end for LFPs?

But capacity test all your cells, see what you have left. You only have 14 batteries to test, so it's easy, all you need is an imax b6 charger or something similar. Your batteries are fairly new, so having to replace 2 out of 14 wouldn't be a bad idea to keep them more balanced.
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#35
(02-19-2019, 10:32 AM)Sean Wrote:
(02-18-2019, 03:41 PM)babish Wrote: Does anyone has a not too expensive shunt trip breaker to recommand?

These, I doubt you'll find a better quality DC breaker at that price - they come with trip signalling contacts, you just need to add a trip (internally) with a coil voltage to suit your control system. Output A pulse from a WM4 works well.

https://www.ebay.co.uk/itm/Merlin-Gerin-...ect=mobile

In my case I guess it would be much cheaper to just use a contactor, eg https://www.ev-power.eu/Battery-Manageme...ctor%2048v
The main issue that worries me at the moment is an over discharging event, but it would also be nice to be protected in case the solar charger fails to stop charging above the absorption voltage level, or in case of any other issue.
So ideally it would switch off the battery when "Critical BattOk" is turned on, but then I understand the solar charger could be damaged if it gets disconnected to the battery while it is still receiving current from the solar panels?
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