THERE ARE MANY installations where two or more chargers may be connected in parallel. What about connecting chargers in series?
When you put two chargers in parallel, you know that the main requirement is that they have the same voltage ratings. They may have different current ratings if load current sharing isn’t important.
But when you want to connect chargers in series, you must be more careful. You may see this situation at a site with two 130 Vdc batteries connected in series to make a 260 Vdc battery.
Why isn’t this just considered a 260 V battery? We should use a 260 V charger.
The reason for the 2 × 130 V designation is usually that the battery is tapped in the center, to provide power for both 130 Vdc and 260 Vdc loads. In this arrangement, the discharge rates on the two 130 V sections will be different. If you tried to charge the two sections in series, using a 260 V charger, one of the 130 V battery sections could be seriously overcharged. Or one could be seriously undercharged. So, users employ a separate 130 V charger for each individual section.
That sounds OK to me. Why the big deal?
It is OK, until you start to add options to the chargers, like ground fault detection. When you do that, you’re creating an ohmic connection between two chargers that should be isolated from each other because of their common connection to the battery midpoint.
Other problems can arise if someone wants to use two existing 130 V chargers to charge an existing 260 V battery that isn’t tapped in the center. Now you really have to be careful: The current ratings must be identical, and the current limit settings on both chargers have to be the same. Even with careful settings, there could be problems in current limit with voltage sharing – the ability of the two chargers to share the battery voltage equally. Equal voltage sharing is crucial to avoid overstressing the chargers’ internal components.
The bottom line: you need, I repeat need, to talk to Engineering before you put two chargers in series.