FNM HPE Series Owner's Manual page 81

Green color: charge level above 65%. No maintenance
required.
Dark color: charge level below 65%. Recharge immediately.
Yellow color: replace the battery.
The data are for information, always read the producer's instructions.
Never try to refill with any type of fluid with this type of battery.
3.3.6.2 Traditional lead-acid batteries
To check the electrolyte level refer to the level marks on the battery side.
To refill:
1. Remove the caps on the upper part.
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FNM HPE Series
Figure 31 - Maintenance free batteries
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