I could do with some help making sure I understand some MPPT battery charging fundamentals. Been reading up on this but can't find anything that has made it 100% clear for me.

  1. An unloaded, fully charged, sealed lead acid battery should read between approx. 12.6 to 12.8V on a multimeter across the terminals;

  2. In a 3 stage MPPT Bulk, Absorption, Float setup - the Bulk charge voltage is generally quoted at around 2.35-2.45V per cell which, for a sealed lead acid battery is around 14.1 to 14.7V;

  3. If you have a charge voltage of approx 14.5V connected to a battery that has been charged to approx. 12.5V - you're going to be reading the charge voltage (e.g. 14.5V) across the terminals, not the battery voltage (e.g. 12.5V);

...and this is my question and the bit that is confusing me - regardless of the charge voltage, the actual battery is only ever charging from approx. 12.6 to 12.8V - it's not actually charging from approx. 14.1 to 14.7V.

Is that correct? I'd appreciate any clarifications or corrections to this. Thanks.

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    \$\begingroup\$ Think about the battery having some resistance in it. When you force current in the voltage is higher. When you take current out the voltage is lower. The battery voltage refers to what voltage the battery would settle at if you stopped trying to charge it. \$\endgroup\$ May 15, 2023 at 16:07

1 Answer 1


What's missing from your description is any mention of current flow.

The charge voltage always needs to be a little above the battery open circuit voltage in order to get current flowing into the battery and thereby achieve charging. Keeping the charge voltage constant at 14.x volts you will see that as the battery fills up the current drops to a small fraction of the initial current. When the battery is completely full and 14.x volts are applied to the terminals, you will still get a tiny bit of current flowing in but the energy going in will just generate heat.

If you have the opportunity, I would recommend playing around with a battery and a variable power supply to observe how the battery responds.


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