# Two Battery Chargers on One Battery?



## Mr. Kingsbury (Aug 21, 2017)

We have an RV with a battery charger installed; we didn't know this until recently. When we had the RV running off a genny we also had a separate battery charger attached to the battery. It occurred to me that this might not be helpful, and possibly harmful.

Thoughts?


----------



## ET1 SS (Oct 22, 2005)

You really need to check the battery manufacturors website specifications for recharge that battery.

Then monitor both the voltage and the amperage being presented onto the battery bank.

There is a possibility that your two chargers are exactly what they need.


----------



## Mr. Kingsbury (Aug 21, 2017)

Very cool. I'll do so.

Thank you : )


----------



## KC8QVO (Mar 22, 2011)

I wouldn't think the battery would be much of an issue.

Battery chemistry is important and that having been said, I am assuming you have "chargers" matched to the chemistry of the battery(ies)? If you have lead-variant batteries (AGM, SLA, flooded, etc) that takes a different (and more common) type of charger. Lithium-variants (LiIon, LiFePO4, etc) are much different. 

However, if you look at stand-alone, off-grid, alternative energy systems that implement large solar arrays or a combination of wind and solar power - the energy goes in to the battery bank before being used directly, or more commonly - through an inverter to get AC power off of that bank. 

Take solar and wind power, a combination system, for example. 1 wind turbine and 1 reasonable solar array will have "2 chargers". One regulates the charge current from the wind turbine. The other regulates the charge current from the solar panels. 

So, your case of "multiple chargers" I don't think is so much a question of the "battery", rather the chargers themselves. Chargers regulate charge current (and voltage, they really go hand-in-hand - each battery chemistry has an "algorithm" that passes the buck between current and voltage maintenance different ways, but they tie together). They also sense voltage so as to not "over charge" a battery (or battery bank).

If your battery bank (whether it is 1 battery or several - I assume you are talking about "house batteries" - what you use, as opposed to a starter battery for an engine) is in-use and is not fully charged, more on the low end of its depth of discharge, lets say, then you can send all the charge current you have between the 2 forms of chargers (reasonable charge current - there is a limit) to the battery bank and it will take it. 

When you get closer to the peak charge state of the battery bank things get tricky. The same theory of the chargers regulating voltage is there, but if your charge current is a significant percentage of the amp-hour capacity rating of the battery bank you will have too much current available for the chargers to really do their jobs.

Lets call it a 12v system and you have 100Ah of battery capacity in your bank, and we'll call it a lead-based battery bank. It may only take 3 amps to "float charge" that bank once it is near peak charge - and that 3 amp charge current will pulse. The voltage will rise in the system (battery bank and charger output - the battery bank draws down the voltage of the output of the charger, while the charger tries to raise the voltage) until the cut off voltage of the charger is reached. At this point the charge current will stop. Batteries will rest at a lower voltage than their peak charge voltage. So once the charge current stops the the battery voltage will drop. Then the charger will return to pushing current to bring the bank back up to the peak charge voltage - if it can (if it is in-use and you are drawing more energy than the charger can provide the voltage will drop, even though the charger is pushing current).

Now lets say that your two chargers combined can push 100 amps. If it only takes 3 amps to effectively float charge at the upper end of the charge state of the battery bank, but you are trying to send 100 amps to it there is no where for the 100 amps to go. As soon as that high current is turned on it grossly sends the system voltage over the peak charge state. The voltage sensing of the chargers will sense this and cut the charge current off nearly instantly. 

This is where lithium batteries differ. When you approach peak charge voltage the charge algorithms switch to a "constant voltage" function where the charger is looking to match the peak charge voltage, and regulates current to meet it. This is different than just sending a lot of current to the battery bank and waiting to see a set point voltage.

If your chargers have various current states between different ranges of voltages, as opposed to being, say, a "30 amp charger" that pushes 30 amps in on/off cycles (say, on the low voltage discharge state it pushes 30 amps, then drops to 10 amps mid-way, and then 5 amps to finish the charge cycle) then you're going to be a lot better off. However, the other charger pushing current at the same time might confuse charge algorithm of the other, and vice versa. 

At the end of the day - as long as the battery voltage doesn't go above its peak safe charge voltage you should be fine. The higher the current the chargers push the tricky'er it becomes. Though, as in combination alternative energy systems - many charge controllers can be used to maintain the same battery bank - its very very common place to do so.


----------



## wy_white_wolf (Oct 14, 2004)

More likely to damage one of the chargers than the battery.

Multiple power sources are commonly used on solar setups. Think solar with a wind Genny or arrays big enough to need 2 charge controllers. But the charge controllers are designed to handle there being more than 1. They might not work as intended if the voltage points are different but won't be damaged. 

That may not be the case with the chargers you used.

WWW


----------

