I have a very special application in mind but do not understand how the circuit need to be functioning in order to determine when batteries need to be replaced.
I will use two LG18650 F1L (3250mAh) batteries in series for a nominal output of 7.2-8VDC. When supplying power these batteries are not (and can not be) connected to anything else than the load (i.e. not to Charger circuit or BMS). The load current is fairly constant at 60-75mA. The load is applied for up to max. 19 hours per day (i.e. use of less than 50% of the new battery capacity), after which the batteries are connected to a charging circuit (to be developed) until they are fully charged or max. 5 hours. This use cycle repeats every day until it can be automatically determined that batteries can not support this cycle and need to be replaced.
I’m looking for suggestions on how a method could best determine when the batteries should be replaced. I need a way to automatically notify the user that the batteries need to be replaced. Is there a way whereby a circuit could to a “test” just before starting to charge and/or a “test” after batteries are fully charged but not yet connected to the load?
I would expect that two high quality batteries (like the ones mentioned above) would last quite a long time in the described use but I still need to figure out a way to determine when to replace them.
Anyone that could offer suggestion for a “method” to accomplish this?