charging battery bank with a mains power charger

Sometimes, I have seen battery chargers "rated" for maximum battery bank storage capacity (say 300 AH @ 12 VDC). There is always the possibility that a very large battery bank could cause the battery charger to overheat (running at maximum current for too many hours).

Another possible reason for "max battery bank AH rating" would be if they are following other rules of thumbs for charger/battery bank design. More or less, the optimal solar charger recommendation is ~5% to 13% rate of charge (i.e., 100 AH 20 hour rated battery bank, 5 amps to 13 amps). 10%+ rate of charge generally works well for full time off grid. And 5% rate of charge is better for weekend/seasonal usage during sunny weather.

Battery bank wise, for flooded cell batteries, 10% is, many times, the manufacturer's minimum recommended rate of charge for deep cycle batteries (optimum for battery chemistry). For mains powered (or genset powered), 10% to 20% rate of charge usually works out pretty well (~25% maximum rate of charge). For typical flooded cell lead acid batteries, over ~13% rate of charge can cause battery bank to run hot--A remote battery temperature sensor is just about required for automated charging--Or monitoring the battery bank (i.e., with a generator, 20-25% rate of charge, run until the battery bank is ~80% state of charge, then let the solar array finish the charging at a lower current).

So, a battery charger rated for a 300 AH maximum battery bank would be charging (at 5%) 15 amps maximum output current. If 13% rate of charge, closer to 19 amps rated output current.

I have charged a "dead" ~85 AH car battery with a 1-2 amp trickle charger (cheap), and the charger did work and bring the battery back to life--But the charger overheated and popped open the plastic cube (charger case). It was pretty surprising (and unsafe if the charger fell onto the metal car chassis). An example of a poorly designed charge controller unable to manage running at (or over) rated output current for several days. I did replace the battery (I find that car batteries that are over ~1 year or so old, tend to die very quickly if they where taken dead and recharged). Also, I bought a better battery trickle charger (Battery Minder or Battery Tender brands).

-Bill

A well designed charge controller in a well ventilated location should not overheat no matter the AH rating of the battery bank. And when you have a larger battery bank, it is not uncommon to parallel two or more charge controllers to get higher total charging current (faster charging, shorter genset run time).

Just make sure you use breakers/fuses inline with the charger to battery bank + cable connections (sized for the wire/cable ratings, and at least ~1.25x the expected max continuous charger current to battery bank). Breakers/fuses (in north America) are generally specified to not blow at 80% or less of rating, and will eventually trip (many minutes to many hours) at 100% or greater of rating.

Charging a large battery bank is a "difficult load" for electrical circuits. You have have the circuit+charger supplying 100% of rated current for many hours (i.e., 10% rate of charge for 20% to 80% charging, or 6 hours). Nominal house wiring is designed because most circuits have variable/low current needs. I.e., a few lights and a refrigerator that takes 600 Watts (say 5-6 amps on a 15 amp circuit, or 80% of 15 amps = 12 amps max continuous) to start or defrost only averages 120 Watts running.

When charging a battery bank, it is very easy to run the charger at 100% of rated output (and input current) for many hours--Which tends to heat the wiring and stress the breakers--And why I like to run battery charging circuits at 80% of rated power (or 1/0.80=1.25x rated continuous load current).

-Bill

I understand. For example (since I do not have any details)... Say you have a 30 amp @ 12 VDC charge controller that runs on 110 VAC mains. Designing the DC wiring (charger to battery bank, put fuse/breaker near battery bank--source of "high current" if there is a short circuit/charger failure):

• 30 amps * 1/0.80 NEC derating (same as multiplying by 1.25) = 37.5 amps ~ 40 amp wiring+fuse/breaker for protection minimum.

I suggest a minimum of 0.05 to 0.10 VDC drop when charging a 12 volt battery bank... Say you have 6 feet from charger to battery bank, one way, 30 amps, 0.10 volts max drop (high voltage drop means batteries look "fuller than they really are" and slows down charging):
http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.8152&voltage=12&phase=dc&noofconductor=1&distance=6&distanceunit=feet&amperes=30&x=88&y=16

4 AWG wire 6 feet (one way run) will have a voltage drop of:
Voltage drop: 0.089
Voltage drop percentage: 0.74%
Voltage at the end: 11.911

Using the NEC table for wire/current ratings (fairly conservative):
https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm
4 AWG is good for 70-95 Amps (at room temperature, depending on insulation chosen--Your breaker would be "legals" at 40-70-95 amps--The 4 AWG wire is "good" for the current)...

And then for the AC input of the charger... Assume 80% efficiency (some chargers are worse, others are better):

• 30 amps * 14.75 volts charging * 1/0.80 charger efficiency * 1/110 VAC mains * 1/0.80 NEC derating = 6.3 amps @ 110 VAC (estimated)

A typical 14 AWG North American House circuit is rated for 15 amps maximum (breaker rating), if you derate by 80%:

• 15 amps * 0.80 NEC derating for continuous current = 12 Amps max continuous...

So, using my above guesstimates, you can run just about 2x of these chargers from one 15 amp circuit @ 110 VAC (it is safe, you are just approaching the "false trip" of a circuit breaker or fuse at ~12.6 amps @ 110 VAC for two 30 amp chargers--You probably would never have any problems--But this is how an "engineer" would approach a "reliable design" with margin of safety factors--legally, you are below 15 amps).

Note that charging connections to battery bank should be ring/lug bolt-up connections. Alligator clamps are only reliable for short term usage--Not good for long term permanent installation (alligator clamps tend to go "high resistance" over time).

Does this long post help/make sense?

-Bill

What voltage is your battery bank?

Connect the charger to your battery bank (less than 80% SOC) and see how hot the battery charger gets after charging for 15-60 minutes of charging.

If the a.c. battery charger does not get hot, you probably are fine. And yes, the battery bank will take longer to charge.

- Bill