# Help understanding RV/Marine battery chargers

I am trying to understand how modern 3-stage marine/RV lead-acid battery charge systems work. It's very common to wire a charger in parallel with both the battery bank and the "house" power. This means that when plugged in, the charger is both charging up the batteries and powering any DC electronics.

My understanding of IUoU Charging is that during the "Uo" phase the charger puts out constant voltage until the current draw drops below a given threshold. If the house electronics are drawing from the charger, then how could the charger possibly know what that threshold is? Shouldn't it be quite common for the charger to never hit that threshold and constantly overcharge the batteries (e.g. if the house refrigerator is in the circuit)?

• That's a very good question. I think there is a timer that keeps on applying that over-voltage up to a set amount of time and then it drops to the float charge level. Unless the charger has a way of comparing the currents in and out of the battery, I wonder how else would it "know". Also, if the draw is larger than the charger's maximum current, there's another question of what does a charger do in that situation? Probably switches back to the constant-current stage. Apr 13, 2020 at 17:28
• @EdinFifić - Right, I was also thinking the charger could turn itself off periodically to measure what voltage the system drifts down to and use that measurement for .... something... It seems like a fairly touchy algorithm though, and it's just strange that I haven't found any mention of the problem in the manuals I've been reading through.
– rcv
Apr 13, 2020 at 18:00
• Apparently, they assume that batteries will be charged out of a circuit or with an insignificant load. Apr 13, 2020 at 18:56
• @EdinFifić do you happen to have a source for that? I couldn't find any mention of limits for parallel loads in any of the manuals I read. Some chargers do have an auxiliary DC output, but even their diagrams show the main inverter just hooked up in parallel with the charger and batteries (e.g. imgur.com/a/h6pGcBh)
– rcv
Apr 13, 2020 at 19:32
• I don't think they would need to mention such limits because you know a charger can't supply more than its maximum current or the current used during the constant-current stage. Apr 13, 2020 at 19:41

When there’s a load, the charger current will be split between the load and the battery. The charger can still monitor voltage at that time.

When the load is removed the charger can revert to the charging profile, terminating to float voltage at the end.

Reapply load and the process starts all over, depending on how much the battery gets discharged. So a refrigerator cycling on and off would make the charger keep switching modes, based on sensed voltage and its calculation of battery state-of-charge.

The charger can (well, could) also monitor current in and out of the battery to help figure out battery state of charge - that is, ‘count coulombs’ as some Li-ion chargers do. But for lead-acid, mainly the voltage method is used. More here: https://batteryuniversity.com/learn/article/how_to_measure_state_of_charge

• Good point, most refrigerators wouldn't act as a constant load as long as their insulation is reasonable - the compressor would turn on only occasionally to keep things cool. If the load never cycles off (e.g. floodlights are left on) then the charger would have no way to determine the difference between the load draw and the battery charging right? In that case, would even modern chargers just stay in the Uo absorption phase?
– rcv
Apr 13, 2020 at 19:23
• Also, in the case of a refrigerator load, it's not like the charger is properly accounting for the load. It's just "not a big deal" because that load will hopefully be on for less time than it takes to overcharge and damage the batteries?
– rcv
Apr 13, 2020 at 19:25