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


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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  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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