Battery size and chargers

[ameyers41]

Boat currently has a Guest model 16123 3 bank charger rated at 4 amps per bank. Just bought a new 24v Terrova to replace the 12v 55lb Powerdrive that came with the boat. I want to go with the biggest batteries possible for the trolling batteries. I think the cranking battery I bought last year was a size 27 (for a 75 yamaha 4 stroke). In looking at the charger user manual for the first time, I see the specs on the charger says maximum battery size 24 or 75 amp hours. The batteries I was looking at for the trolling motor were at least 175 amp hours and again, size 27.

Is this a big no no? Do I need to upgrade chargers? Will it burn out the charger, or worse, the batteries prematurely?

[aanderud]

You won't burn anything out but if you have a 4 amp charger, but it's going to take you 40 hours to charge a 175 amp-hour battery from empty to full.

By the way, where are you finding 175 amp hours on a size 27? I mean what brand? The batteries I have are size 29 and they're at most 120 amp hours. I was at interstate batteries HSOforum and their size 27 batteries come in two capacities -- one of them gets roughly 75-100 amp hours (20.4 hours at 5 amps, but only 3 hours at 25 amps), and the other gets roughly 65-85 amp-hours (17 hours at 5 amps, and 2.6 hours at 25 amps).

They do have a size 4D battery that will supply 25 amps for 6.5 hours (which is about 175 amp hours), but that sucker weighs 119 lbs (and it is 9 inches longer, 1.5 inches wider, and 1 inch taller than the group 27 battery)

[ameyers41]

My apologies. I read the labels wrong when searching the other day. And now that I look at my notes, I think I was thinking the number of the reserve capacity, not amp hours...

I'd really like to get the biggest batteries I can without having to replace the charger for now. I'm not worried about recharge time as much. I think my biggest concern is the note in the user manual stating "battery size may be large enough to cause the battery charger to not transition to float stage due to the batteries internal resistance."

For those electrical experts out there, is that a real possibility with higher capacity size 27s or size 29s?

Thanks again!

[BobT]

I could see it as a potential. In another thread I gave an example where setting my charger to the trickle setting (2A) it would never see the battery as fully charged because the battery's internal resistance would never get high enough. I discovered the problem when I seemed to find my electrolyte levels going down all the time. I began using the 15A setting and the problem went away.

[ameyers41]

BobT - What size batteries were you experiencing this with and how big is your charger?

[BobT]

The charger is a Schumaker multi-purpose charger. I can set it for regular or deep cycle automatic charging and I can select between 2A and 15A settings. There's also a 100A starting setting that I've never used.

My deep cycle battery is a Stowaway 105AH battery.

[aanderud]

I could see it as a potential. In another thread I gave an example where setting my charger to the trickle setting (2A) it would never see the battery as fully charged because the battery's internal resistance would never get high enough. I discovered the problem when I seemed to find my electrolyte levels going down all the time. I began using the 15A setting and the problem went away.

BobT, correct me if I'm wrong, but I believe it's a HIGHER internal resistance that causes the battery to read at a LOWER voltage, which causes the float voltage never to be reached. Most chargers will transition to float at something like 13.8 volts, and if your internal resistance is too high, they might not read high enough to transition.

Everything I know about batteries has indicated that having a LOW internal resistance is desirable. In fact, some batteries have such a low internal resistance that you can safely charge them with 100amps or more without creating too much heat. The internal resistance times charging current equals watts of dissipated heat, so you can see where high internal resistance can create a problem when charging.

[BobT]

Think of it this way. What voltage would you read across a load at zero resistance? Zero volts. What is the resistance between your tester leads when they are not conected to any load? Infinite.

Typically, a battery's internal resistance will drop with age and temperature. Heat and age are not a battery's friend. I suspect that a low internal resistance would depreciate a battery's ability to maintain a charge as it would discharge within itself wouldn't it?

I'm just applying what I understand about electrical theory. Power supplies, i.e., Batteries, may act differently and so maybe I'm wrong.

[aanderud]

Yes, you're wrong. Internal resistance is a BAD thing. And it INCREASES with age. You want less of it.

[BobT]

Yes, you're wrong. Internal resistance is a BAD thing. And it INCREASES with age. You want less of it.

I must have been thinking backwards. I think my line of thought was along the lines that if the resistance was low, the battery wouldn't be able to develop a potential. It makes sense that a battery having a high internal resistance will not be very capable of taking or delivering current and therefore unable to accept a charge. In fact, the lower the internal resistance of a battery, the faster it can be recharged and the more current it can deliver. I suppose this is one of the differences between a starting battery and a deep cycle battery. I appreciate the correction.

[Whoaru99]

Yes, lower internal resistance generally means the capability to source more current, but it's that same lower internal resistance that could keep the (apparently) current sense circuit in the charger from shifting it to float mode because charge current never drops below the threshold.