How can you tell if a battery is fully charged?
The only accurate way to tell if a VRLA DRY CELL AGM or GEL battery is fully charged is by using a good voltmeter to determine the open circuit voltage (OCV) without any load applied to the battery. Accessible flooded-type batteries can also use a hydrometer.
Table 5 - State of Charge vs. OCV
|
Charge % |
Open Circuit Voltage Flooded Battery |
Open Circuit Voltage GELL Battery |
Open Circuit Voltage AGM Battery |
|
100% |
>12.60 |
>12.85 |
>12.80 |
|
75% |
>12.40 |
>12.65 |
>12.60 |
|
50% |
>12.20 |
>12.35 |
>12.30 |
|
25% |
>12.00 |
>12.00 |
>12.00 |
|
0% |
<11.80 |
<11.80 |
<11.80 |
Divide the above values in half for 6-volt batteries or by six to determine cell voltage. The TRUE OCV can ONLY be measured after the battery has been removed from the charge or discharge load for 24 hours.