# Battery Charger using LM317

I'm designing a battery charger consisting of an LM317. The regulator will charge three batteries successively, for example, when battery 1 reaches a terminal voltage of 13.8, a voltage comparator will activate a relay that will switch on the second battery while switching off the switch for battery 1. The same will happen to the third battery when the second battery attains a full charge.

Also, all the switches are paralleled with a resistor so that a trickle charge will compensate for the self-discharge of the battery when the switch for that battery has been turned off.

For a 12V 10Ah, with the configuration attached below, here is my expected current to be delivered to the battery that is currently charging,

I = (Vs - Vb) / 1 ohm

I have researched the state-of-charge voltage level of the battery, based on a 0.1C charge rate:

80% 13.8V
60% 13.4V
40% 13.1V
20% 12.8V

(these are just approximations, the paper where i read this said that the average error is less than 5% )

therefore, as the battery voltage increases, the charging current decreases:

Imax = (13.85 - 12) / 1 = 1.85A
Imin = (13.85 - 13.8) / 1 = 50mA

so that means that from the moment the switch is turned on for the current battery, 1.85A is being delivered, and then when the 80% charge has been reach, only 50mA is being delivered. The remaining capacity will be fully charged by trickle charge.

my question is, am I correct with everything I said?

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