lifepo batteries

Things 12V and 24V
dennisail
Posts: 61
Joined: Tue Feb 19, 2013 1:42 pm

Re: lifepo batteries

Post by dennisail » Tue Sep 06, 2016 5:16 pm

Sounds like a simple and effective system. Do you have anything in place to protect the system if say one cell develops a short? How have you configured your cells? 14.4V Seems a little high for longevity. Have you played with measuring the capacity at slightly lower voltages? 14.4 is very close to max charge, seems a lot of extra voltage for very little extra capacity compared to charging to 14V. (at least from published data I have read).

If you are only measuring the total voltage for your system protection, you may have an issue if one cell develops a short/partial short. Lets say for arguments sake the bad cell goes to 1V. Its low voltage will drag any parallel cells down with it possibly taking them out too (undercharge). The overall voltage may still look OK to your charge controller at 14.4 volts. But 3 cells will have 4.46V on them and the bad one only 1V. Have you done anything to protect the system from something like this occurring?

Bedar was close to exploding due to exactly this happening with a lead acid battery. When I got the boat the batteries were toast. I had a few spare car batteries and put a few in. One cell shorted out. The voltage stayed at 14.6V, but with one shorted cell, the cell voltage on the others was very high. High enough to cause thermal runaway and boil the battery with my high power solar array. The boat stunk of rotten egg and the battery was boiling. There would have been bulk hydrogen and oxygen gas in the battery area, if it boiled all the way down it may have sparked and ignited.

Lithium will behave differently to that if one cell shorts out, but the outcome may be similar without something to stop that scenario from occurring. Individual cell monitoring seems the most obvious way as a safe guard rather than total bank voltage. "BMS" is just a buzz word really. You could easily make something for fairly cheap from circuits off ebay and configure it so it works the way you want, unlike an off the shelf BMS.

This little circuit costs $6.65 delivered, it will trigger a small relay (connected to your main relay). You can program both the high and low voltages you want it to trigger at in the module and has a V display board too.

http://www.ebay.com.au/itm/Voltage-Moni ... SweuxWQxTK

44c
Posts: 1146
Joined: Tue Aug 08, 2006 5:08 am
Location: Hervey Bay, Qld

Re: lifepo batteries

Post by 44c » Wed Sep 07, 2016 7:33 am

EV works suggested 3.5-3.6 volts per cell is a safe charging voltage. So that's 14 - 14.4 volts.

I'm at the top end, again it's just the habit of trying to tuck every amp away when the sun shines. I've dropped it to 14.2 volts, right in the middle of the safe band. In reality though the charge controller only hits the set voltage for a moment, then goes into float. I doubt if there's any real risk.

As far as individual cells going bad, I keep monitoring them. From all I've read, it's actually less dangerous with LiFEPO4 than it is with lead acid.

dennisail
Posts: 61
Joined: Tue Feb 19, 2013 1:42 pm

Re: lifepo batteries

Post by dennisail » Wed Sep 07, 2016 8:10 am

I agree the dangers as in risk to health/the boat due to explosion are less with a shorted cell. But the risk to the wallet are greater :lol:

I think the higher charge voltage thing is not so much a risk, its just a longevity thing. I read one blog where someone followed the "guidelines" and charged to the max recommended (by winston) charge and had a life span less than lead acid. But the exact voltages escape me now. I think they were going over 14.4 though.

I come from a control systems background, so designing a cheap fail safe system to guard from all possible failure modes is actually kind of fun for me. I just like tinkering with that stuff. I don't know what the chances are of a cell developing low voltage/short due to a cell failure, or how fast it could happen. I just know what could happen to the rest of the system if it occurs when a controller is allowed to maintain the overall voltage.

Have you counted the amp hours between the controller hitting 14.4v and discharging to 13.3v? I would be interested to learn how much capacity that is, and what the difference is between that and the 14.2v setting.

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