A low-voltage disconnect for 12 volt lead acid and lithium batteries

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KA7OEI writes:

The avoidance of overcharging is usually pretty easy to avoid: Just use the appropriate charging system – but overdischarge is a bit more difficult, particularly if the battery packs in question don’t have a “protection board” with them.
Lead acid batteries (almost) never come with any sort of over-discharge protection – one must usually rely on the ability of the device being powered to turn itself off at too-low a voltage and hope that that threshold is sensible for the longevity of a 12 volt battery system.
Many larger (e.g. >10 amp-hour) lithium-iron phosphate (LiFePO4) do not routinely come with “protection” boards unless it is ordered specially: Such batteries include some of those “Lead Acid” replacements and some of the more “raw” LiFePO4 batteries available from many vendors, such as the 20 amp-hour modules made by GBS.
While it is also important to equalize LiFePO4 batteries when charging (refer to this post – Lithium Iron Phosphate (LiFePO4) batteries revisited – Equalization of cells – link) the more immediate danger in routine use is accidental over-discharge.
For lithium batteries, one may install “protection” boards that prevent accidental over-discharge and, in some cases, provide charge equalization – but such things are much rarer for lead-acid batteries, but such a circuit is quite simple and is applicable to either Lithium or Lead Acid batteries.

More details at KA7OEI blog.

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2 Comments

  1. As with many such articles, the author is making assumptions on usage that are not true in many cases. For my usage, LiFePO4 are much harder to overdischarge than Lead-Acid, and IMHO cell auto-balancing or poorly designed monitoring are more likely to cause problems than solve them. His problem was more due to charging to to high a voltage rather than lack of cell balancing. (At less than 1/2 C rate, 14.0v or lower should be used as a maximum charge voltage of a 4 cell pack.)

    My blog has pointers to more info, and cruisersforum has a huge thread on them.

    1. Agree, he seemed *really* concerned about LiFePO4 over-discharge. I would be really interested about his experiences that prompt such concern. I ran one 14500 LiFePO4 cell well under 2.5V with a load of about 5mA running a *ahem* buggy PIC16 program, it survived and capacity seems fine *touch wood*. Only issue I had was one dead on arrival cell, and that the discharge curve past 3.1V is really steep…

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