CONTINUOUSWAVE

I have two 12-Volt deep [cycle] marine batteries in series and a 24-Volt charger that is supposed to taper down and stop when the batteries are fully charged.

Q1: What happens if one battery is weak and takes a long time to charge or never gets to full-charge?

Q2: Will the charger keep going and possibly burn out the good battery?

Q3: Or, will the charger stop when the good battery is full-charge and leave than other one undercharged?

When batteries are connected in series for charging, the charging current is the same in each battery. If the charger supplied 1-Ampere of charging current to the series-connected two 12-Volt batteries, then each battery will get 1-Ampere of charging current passing through it.

To answer the questions several assumptions must be made:

• the battery charger only has two connections, a positive and negative, and cannot tell the voltage across individual batteries in the two-battery series string
• the charger determines the state of charge of the series connected string by the total voltage of the two batteries
• the charger's algorithm for apply charging current is based on the voltage across the two batteries

With those assumptions, I will propose these answers:

Q1: What happens if one battery is weak and takes a long time to charge or never gets to full-charge?

If one battery needs more charge than the other, its terminal voltage won't rise as fast. Since the charger only sees the total terminal voltage of two batteries in series, it won't see a terminal voltage that it thinks is indicative of full charge until both batteries get to full charge. It is also possible that the charger might see enough voltage to stop charging if the more-charged battery has an unusually high terminal voltage.

Q2: Will the charger keep going and possibly burn out the good battery?

The charger will continue to apply charging current until the total voltage of the two batteries reaches the threshold that the charger thinks is representative of full charge. This suggests that if one battery is already at full charge, it will continue to receive more charging current. To know if this will cause harm to the battery, we must know more about the battery. If the battery is a flooded-cell vented design, the over-charging will just tend to boil away electrolyte. If the battery has a sealed case and is designed not vent to the atmosphere unless there is dangerous pressure, the battery may or may not vent electrolyte, depending on what the release valve pressure is set for and the pressure build up.

Q3: Or, will the charger stop when the good battery is full-charge and leave than other one undercharged?

The charger will stop when the combined voltage reaches its threshold. If the batteries have unequal terminal voltages, the charging could stop when one battery is charged beyond full charge and has an unusually high terminal voltage, leaving the other battery with a lower terminal voltage which could mean lower state of charge.

Regarding trying to charge batteries connected in series, the general rule for a good outcome is all the batteries must have very similar characteristics, and the more identical in every way the better. A uninterruptible power supply (UPS) system that I managed used 40 12-Volt batteries in series. The nominal full-charge float voltage was about 13.2-Volts, giving a total string voltage of 528- to perhaps 540-Volts DC. The system included a sophisticated individual battery monitor that measured the effective internal resistance of each battery. As the string of batteries aged, the internal resistance of some batteries would tend to rise. Those batteries were removed from the string and replaced when their internal resistance became too divergent (too high) from the others in the string. When more than ten-percent of the string's batteries were in need of replacement, all 40 batteries were replaced. This was due to the service: the UPS was to provide power for critical systems in the event of failure of the utility power, and to last long enough to allow a local generator to start and come on line.

If the service is less rigorous, as in powering a fishing motor, you can tolerate a higher probability of failure.

In a string of batteries, the resultant reliability of the string is only as good as the worst battery in the string. For this reason it is generally preferred to keep all batteries in the string as identical as possible. It goes without saying that the batteries MUST be of the same manufacturer, same model, same age, and have only been used in the series string to provide power at the total series voltage. Any other configuration will cause charging the string as a single battery to produce undesired results.