Battery charger size needed

badrano

Petty Officer 1st Class
Joined
Aug 7, 2018
Messages
328
I've been reading up on how to determine what size charger is needed. Some things I've read is to take the amp hour rating and multiply by 10%. Based on this, it looks like I would need a 20 amp charger (10 amps per channel - 2 batts). But then I've also read that I wouldn't need a 20 amp charger because I don't need the ability to recharge quickly and that a smaller charger could be used. The reason behind not needing the ability to recharge quickly is because the boat sits idle for a week or so between outings.
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
26,826
Two batteries? Figure on 5A each, for slow charging, which is the best kind.

A 10A charger will normally taper the amperage as the batteries charge up, so you will not overcharge them.

You can also get a smaller charger, if you can leave them on charge for a day or two.
 

dingbat

Supreme Mariner
Joined
Nov 20, 2001
Messages
15,128
It's detrimental to the life expectancy of a battery to sit discharged for any length of time.

Minimizing depth of discharge and recharging the battery ASAP is paramount to longevity of a battery
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,781
I have used Smart Chargers the past 10 yrs, as they will recognize/charge an AGM. They are usually 2/8/20 Amp models. I prefer not to use the Battery Boil setting
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
26,826
Gee, this is like an oil debate. I like the slow chargers for boat batteries. Slow is best, as my pappy taught me. The motor charging system keeps 'em charged when I use the boat. When I am a slacker, and don't use the boat, I slap a 3A charge on it for 24 hours, or 48 hours for a dual.

Sometimes I put a video game charger (13.5VDC, say 750 Ma or 1A) on them over the winter. Leave it plugged in for a week. It keeps the battery warm, but only charges when she is real low.
 

airshot

Captain
Joined
Jul 22, 2008
Messages
3,791
If my batteries are really low, I go with a 4 or 5 amp charger to get them back up fairly quick, then switch to a small trickle/ maintainer charger and just leave than on until ready for use. If the battery is only 30% or less used ( still holds 70% charge) I just use my maintainer charger only. I typically get 5-6 years from my batteries, currently have 4 years on my current batteries (4 ) and still going strong. If your batteries are really low then a 5 amp or bigger to get them back up fairly quick then switch over to a trickle/ maintainer to maintain the full charge and get rid of sulfation on the plates. Works for me and has for a good many years.
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,781
The Battery that came with my Boat, Marine Interstate, still had 75% of its Capacity after 14yrs of use. It did cause some attention at the Interstate Distributor when they tested it, then kept retesting it...
 

KJM

Lieutenant Junior Grade
Joined
Jul 31, 2016
Messages
1,133
It's detrimental to the life expectancy of a battery to sit discharged for any length of time.

Minimizing depth of discharge and recharging the battery ASAP is paramount to longevity of a battery
I thought deep cycle batteries were made to be able to withstand having a low charge without harm?
 

KJM

Lieutenant Junior Grade
Joined
Jul 31, 2016
Messages
1,133
Why do you guys use a charger? The engine charges the batteries and they should hold a charge over the winter, shouldn't they? If they go too low just boost it to start the engine and let it run to recharge them?
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,781
We are repeatedly told that the Alternator isn't there to charge the Battery, but to cover the Load while the Car is driven. That never made much sense, However, a lot of Alternators can put out very large amount of Current(140+ Amps, often 80+ amps at Idle), and can damage a battery, if it is severely discharged. Boosting a Dead Battery, will certainly have the Alt sending as much as it can at any given rpm, and will certainly shorten the Battery's Life.
Batteries do tend to discharge over time, so a couple of hours, a few time times over the winter on a Trickle Charger is all that is needed to maintain it.
Leaving the battery in the Boat over the winter, isn't a wise move, cause if it freezes and cracks, the bilge gets an Acid Bath.
 

KJM

Lieutenant Junior Grade
Joined
Jul 31, 2016
Messages
1,133
We are repeatedly told that the Alternator isn't there to charge the Battery, but to cover the Load while the Car is driven. That never made much sense, However, a lot of Alternators can put out very large amount of Current(140+ Amps, often 80+ amps at Idle), and can damage a battery, if it is severely discharged. Boosting a Dead Battery, will certainly have the Alt sending as much as it can at any given rpm, and will certainly shorten the Battery's Life.
Batteries do tend to discharge over time, so a couple of hours, a few time times over the winter on a Trickle Charger is all that is needed to maintain it.
Leaving the battery in the Boat over the winter, isn't a wise move, cause if it freezes and cracks, the bilge gets an Acid Bath.
So why don't car batteries freeze?
h as it can at any given rpm, and will certainly shorten the Battery's Life.
Batteries do tend to discharge over time, so a couple of hours, a few time times over the winter on a Trickle Charger is all that is needed to maintain it.
Leaving the battery in the Boat over the winter, isn't a wise move, cause if it freezes and cracks, the bilge gets an Acid Bath.
 

KJM

Lieutenant Junior Grade
Joined
Jul 31, 2016
Messages
1,133
We are repeatedly told that the Alternator isn't there to charge the Battery, but to cover the Load while the Car is driven. That never made much sense, However, a lot of Alternators can put out very large amount of Current(140+ Amps, often 80+ amps at Idle), and can damage a battery, if it is severely discharged. Boosting a Dead Battery, will certainly have the Alt sending as much as it can at any given rpm, and will certainly shorten the Battery's Life.
Batteries do tend to discharge over time, so a couple of hours, a few time times over the winter on a Trickle Charger is all that is needed to maintain it.
Leaving the battery in the Boat over the winter, isn't a wise move, cause if it freezes and cracks, the bilge gets an Acid Bath.
So why don't car batteries freeze?
 

KJM

Lieutenant Junior Grade
Joined
Jul 31, 2016
Messages
1,133
Oh they do, the less charge in them, the lower the Temp. , 2 winters ago, I had two Freeze in my cars
We must not get that kind of cold here, never heard of it before. The coldest it gets here in winter is about -15C
 

airshot

Captain
Joined
Jul 22, 2008
Messages
3,791
I thought deep cycle batteries were made to be able to withstand having a low charge without harm?
They can, but need to be recharged asap. The longer a battery sets with a low charge, it will shorten its life. Even deep cycle batteries should not go below 50% of full charge where an auto battery should not drop below 20% on a regular basis. The LifePo4 can drop down 80% without a problem.
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,781
We must not get that kind of cold here, never heard of it before. The coldest it gets here in winter is about -15C
-15C is like the Tropics to me, it's not even 0F. -30C, and colder is what my winters are
 

KJM

Lieutenant Junior Grade
Joined
Jul 31, 2016
Messages
1,133
-15C is like the Tropics to me, it's not even 0F. -30C, and colder is what my winters are
Yeah, it used to get a bit colder when I was a kid, but winters are very noticibly warmer here the last 15-20 years. There are winters now that ice on ponds isn't safe until Feb. But I guess thats getting a little off topic.......
 
Top