battery charger amps for two banks

[HerbertFriedman]

I need to choose a battery charger. I have two banks, a house bank consisting of two 100 amp-hr batteries in parallel and a starting battery of about 80 amp-hours, all batteries are lead acid flooded type. What is a appropriate amperage for the charger, is it based on the smaller bank so as not to overcharge or the total amp-hours. Choosing a small amperage charger to save money, say 10 amps, will that harm the batteries or just take a little longer to charge?

 

[HerbertFriedman]

Second part of that charger problem, I had a charger/house bank failure and need to know how to test both systems. During a dripless shaft replacement, the present unit is 18 years old , the tech noticed a crackling sound from the nearby battery compartment. He found one of the two house batteries bubbling water at top and smelled probably sulfur. He surmised that the battery charger was overcharging and cooked the battery, immediately shut off the charger and flooded the area with baking soda, right thing to do. But unfortunately he did not check the lights on the charger to see if the charger was still in standby or float or other charging modes. So the problem could be a failed battery cell requiring more charging current (leading to the flooded condition) OR the charger switched from standby to a charging mode and overcharged the battery, OR both.

The cost of replacing the charger and the house batteries, being the better part of $1K, how could I test both. Probably the battery is shot and needs to be replaced, and probably both should be replaced so they are "matched". The batteries are only three years old, and the charger 10 years.

 

[Frank Langer]

I have a similar battery set up to yours and have a 40 amp charger which works well. In the past I had a 30 amp charger for the same setup and that was fine too, but 40 amp charger will charge faster. Both were smart chargers with three stage charging, so batteries were not over charged.
Frank

 

[Loren Beach]

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This site is great for getting "educated" on many technical boating matters, and charging is covered in several places.

We have a (approx) 260 AH house bank ( two 6 volt GC flooded) and an emergency AGM spiral cell 12 volt battery. Both banks have been attached to a modern multi-stage solid state charger for a decade with no problem. Ours is a 20 amp (IIRC) and a 30 would perhaps be better, in 20/20 hindsight.
Note that I am not advising anyone else as to what they should do, but only relating what works for us.....

 

[HerbertFriedman]

From the two responses, it appears that while a 30 or 40 amp charger might be OK for the house bank ( with total capacity in excess of say 200 amp-hr), does such a high amperage does not harm the smaller "starting" battery which might have a capacity of under 100 amp-hr? I find this strange but then again, I do no know how these chargers actually work.

For example, with two banks of quite different capacities, how does the charger know when to enter the slower charge mode and then the float stage and finally the standby stage. Is that decision based on the larger or small capacity bank? Is the amperage rate the same for both banks so that the house bank dominates and why does that not overcharge the starting battery. Finally, when the indicator light on the charger switches from one mode to another, do all the banks switch. Essentially, the question is: what is the logic for chargers with multiple bank outputs?

 

[Frank Langer]

From the two responses, it appears that while a 30 or 40 amp charger might be OK for the house bank ( with total capacity in excess of say 200 amp-hr), does such a high amperage does not harm the smaller "starting" battery which might have a capacity of under 100 amp-hr? I find this strange but then again, I do no know how these chargers actually work.

For example, with two banks of quite different capacities, how does the charger know when to enter the slower charge mode and then the float stage and finally the standby stage. Is that decision based on the larger or small capacity bank? Is the amperage rate the same for both banks so that the house bank dominates and why does that not overcharge the starting battery. Finally, when the indicator light on the charger switches from one mode to another, do all the banks switch. Essentially, the question is: what is the logic for chargers with multiple bank outputs?

Our smart charger charges all batteries to 80% charge, then slows to lesser charge, then monitors and restarts as needed. It charges the least charge batteries first, then others to bring them to the same level.
Frank

 

[Kenneth K]

I put in 30A Sterling charger in 2021. Sterling is the British version of the US-sold Promariner Pronautic (mine is a 1230p). When first activated (after "sensing" the batteries) the charger will show what percentage of output, 0-100%, it's putting out. I've never seen mine put out more than about 75%. These smart-chargers have a temperature lead the attaches to a battery in your primary bank to prevent battery overheat during charging. They also have variable speed cooling fans that vary with charger output and temperature. So, yeah, they are pretty smart. They won't let you cook or overcharge your batteries

I have a 2X80Ah + 1x80Ah bank setup. At 75% output, I'm only using 22.5 A max, but the charger usually rapidly drops off from 75% to something much lower. You'd probably only ever see max output for a heavily discharged bank. Then, the 40A charger simply recharges quicker than the 30 or 20A.

One other thing to think about is whether you are running other DC items while recharging. If you keep your fridge on all summer, the charger has to power the fridge while it is also charging the batteries. That might be the only reason I'd go above 30A on a charger with my setup.

ProMariner 1230P: https://www.promariner.com/en/p/63130/ProNautic-1230P

 

[Kenneth K]

When the indicator light on the charger switches from one mode to another, do all the banks switch. Essentially, the question is: what is the logic for chargers with multiple bank outputs?

No. They are not that smart. They can only be in any one charge mode at a time, across all banks. I think the logic is "First, do no harm." If one bank is severely discharged and the other is fully charged, I suspect the charger will choose to under-charge (charge more slowly) the depleted bank (which is not hurting the depleted bank, just charging it slower than what's optimal) rather than risk overcharging the healthy bank (and, thus damaging it).

But, if you know this is the case with your banks (it shouldn't be the normal case), you could disconnect the charge leads from the healthy bank until the weak bank is brought back to normal. I do this sometimes with mine as I have kill-switches installed at each bank so I can isolate any bank from the boat's charge/discharge system. This method becomes more important under solar charging, where the total charging output may be limited.

 

[Christian Williams]

Twenty-amp smart charger here, for three AGMS (2 house, 1 emergency start). I'm always charging at the dock, running the 12v refrigerator.

As I understand it, 30- or 40-amp chargers just do the job faster, which is probably of little consequence to most of us.

 

[Mr. Scarlett]

I use the ProMariner wired as shown in Loren's link, with an ACR. Works great. Unless you're only plugging in for a couple of hours before going out again, it's not necessary to go too big. Overnight will be enough to top up most boats without electric motors.

 

[HerbertFriedman]

I put in 30A Sterling charger in 2021. Sterling is the British version of the US-sold Promariner Pronautic (mine is a 1230p). When first activated (after "sensing" the batteries) the charger will show what percentage of output, 0-100%, it's putting out. I've never seen mine put out more than about 75%. These smart-chargers have a temperature lead the attaches to a battery in your primary bank to prevent battery overheat during charging. They also have variable speed cooling fans that vary with charger output and temperature. So, yeah, they are pretty smart. They won't let you cook or overcharge your batteries

I have a 2X80Ah + 1x80Ah bank setup. At 75% output, I'm only using 22.5 A max, but the charger usually rapidly drops off from 75% to something much lower. You'd probably only ever see max output for a heavily discharged bank. Then, the 40A charger simply recharges quicker than the 30 or 20A.

One other thing to think about is whether you are running other DC items while recharging. If you keep your fridge on all summer, the charger has to power the fridge while it is also charging the batteries. That might be the only reason I'd go above 30A on a charger with my setup.

ProMariner 1230P: https://www.promariner.com/en/p/63130/ProNautic-1230P

I have a similar setup, house bank is 2x100Amp-hr and a starting bank of 1x75Amp-hr with a 2013 1230P. Yesterday, at haulout, tech notices one of the house batteries crackling noise and leading water/acid. Unknown if yellow standby light was on or green charging mode but battery is probably toast. Any idea how to check the charger? The price difference between 3 and 3 bank chargers is large, I only need 2 banks. The largest 2 bank charger is 15A so charge will be slower but are there any other downsides?

 

[HerbertFriedman]

I use the ProMariner wired as shown in Loren's link, with an ACR. Works great. Unless you're only plugging in for a couple of hours before going out again, it's not necessary to go too big. Overnight will be enough to top up most boats without electric motors.

So the largest 2 bank charger is only 15 amps, so charging will take longer but are there any other downsides to this choice?

 

[Mr. Scarlett]

The article linked above suggests sizing a charger based on a ‰ of your bank size. Probably not necessary if you regularly plug in overnight. After all, it's not going to help you at anchor.

What charger are you looking at?

 

[ConchyDug]

I just went lithium but if you want I can sell you my old LA charger it does multi banks has a temp sensor, multi-stage, and multiple LA battery types... can also adjust temp settings.

Boaters’ Resale Shop of TX 2307 1447.02 TRUECHARGE 20+ BATTERY CHARGER 20 AMPS . The Boaters' Resale Shop of Texas

Boaters’ Resale Shop of TX 2307 1447.02 TRUECHARGE 20+ BATTERY CHARGER 20 AMPS

www.boatersresaleshopoftexas.com

I have it listed at a local consignment shop.

 

[Andy]

Given your battery bank total 280ah, a 30amp smart charger would be appropriate. They won't overcharge even if you got a 50+ amp charger (so long as you have the right battery chemistry set).

We've got a 1 starter 60ah, 1 house 80ah flooded setup, so 140ah total. To charge, we use a 15amp smart charger with the banks connected via a dual voltage sensitive relay (VSR) and fuses. Here's what we use for reference- Charger + VSR

 

[Kenneth K]

The cost of replacing the charger and the house batteries, being the better part of $1K, how could I test both.

Well, you said you have two other batteries that you don't think are damaged. Hook the charger up to each battery, one by one, and see if it charges normally.

 

[Jerry VB]

If you connect a known good battery to the charger, the charger should raise the battery voltage to 14.1-14.8v (typically closer to 14.2v for marine multistage chargers). Use a multimeter to measure the voltage at the battery for an accurate measurement. The charge current will be relative to the state of discharge of the battery; 2-5A for a battery that is not heavily discharged, the max current the charger is rated for for a discharged battery. This is the "bulk charge" stage.

If the charger raises the battery voltage above 14.8 (or so) volts, the battery charger is defective and will boil your batteries.

If you have a flooded lead acid battery with caps, pop the caps off. If any of the cells are bubbling, the battery is bad or going bad - those cells are not accepting the charge properly.

If the charger is good, after a couple of hours the voltage on the battery will drop to 13.0-13.8v with a smaller amount of current. This is the "float" charge stage.

Addendum: Different battery chemistries (e.g. flooded, gel, AGM) take different "bulk" and "float" voltages. You should verify your battery charger is configured for the correct battery chemistry or it might overcharge (bad) or undercharge your batteries.

Ref: Battery Charging Tutorial

 

[peaman]

I have a similar setup as yours and was recently faced with the same question. Guidance here all seemed to point to 30 or 40 amps for "faster charging", but I chose a 20A charger and am satisfied with it.

Of course it depends on how you use your boat, but in my case, when I'm in my slip and not sailing, charging can be spread over many hours until I sail again. I can't imagine any non-emergency scenario where I would need to fully charge anything in less than several hours.

 

[David Vaughn]

So the largest 2 bank charger is only 15 amps, so charging will take longer but are there any other downsides to this choice?

We have a multi-bank charger and while it is only a max of 10amp to any one battery, I haven't noticed a downside. Especially since it has individual profiles for each battery, so my LiFePo house batteries and AGM starter battery have the charge profile each needs. It has a profile for flooded lead acid as well.
It's kinda cool, in a techie sort of way, to see the AGM battery that is rarely used, charging at a minimal float level or not at all, while the house batteries are taking everything that charger can provide.
To be honest, we rarely have to use it now that the solar panels are in. But I'm pleased with its performance.

 

[Nick J]

David,

I was just responding with my setup (ProNautic 1220p connected to the start battery and a 50 amp DC to DC charger between teh start battery and house bank) and noting I chose it because I couldn't find a charger that used unique charge profiles for different banks. What charger are you using?