Question about batteries

[Joe Dirt]

Last time I used the boat was on July 4th weekend and my batteries were fairly new at the time. I went down to the boat today and they were drained. All the isolator switches were off and nothing was on. Can batteries drain that fast or do you think something is draining them? Seems odd that new batteries would drain in a month. I have an onboard charger and hooked it up this afternoon. I'm worried that something is draining them and not sure if I should go out or not tomorrow.

[wingless]

Idle batteries drain about 10% per month, more at higher temperatures.

Batteries should not remain at less thank 50% state-of-charge to limit sulfation, a permanent damage resulting in reduction of battery capacity.

The battery capacity is a specified ampere-hour rating. A battery is typically end-of-life when the capacity decreases below 50% of the specified value.

There are two easy ways to determine the capacity. In all cases, a fully-charged battery is one connected to an appropriate charger for longer than 24 hours.

Use a long-term load, the best is one that would discharge the fully-charged battery, that has sat idle for 24 hours, from an initial voltage of 12.5 VDC to a final voltage of 10.5 VDC in 20 hours. Other rates are okay. I use my 'fridge that pulls about 7 A continuously.

The other easy technique is to use a Midtronix battery tester.

Those old-school high-load high-rate battery discharge loads are fortunately going the way of the dinosaur.



So, if your batteries are bad, that is easily tested.

If your boat is sucking more juice than expected, that may be measured too. Don't make that measurement w/o following safe procedures.

[Joe Dirt]

The radio and electronics worked but the motors would not turn over. My gauges were reading 12 volts. I'll check in the morning and see what happens.

[wingless]

Twelve V is about 75% capacity. So, there's either a high resistance path, or the capacity is shot. Or a bad solenoid or starter.

What's the voltage read during "cranking"?

[Joe Dirt]

Sorry I can't answr that, the marina pulled the boat and was going to fuel up for me but would not crank. When I turned on the electronics and radio the voltage started going down to like 11 volts, this is all while I had the charger plugged in. It would come back up to 12.1 volts and stay there. I was there for about 30 - 45 minutes with no movement while on the charger.

[wingless]

If a 12 V battery drops to 11 V w/ just the electronics on then no way will it crank an engine. Time for new one(s). I just swapped my six group 31s. That sucked. The new ones are great.

Find a way to plug 'em in for more than 24 hours, at least once per month, to reset the state of charge to 100%. Batteries last longest when they remain fully charged, always plugged into a smart charger is best. They are also rated for the quantity of discharge cycles, so limiting usage maximizes life time.

[Joe Dirt]

Wingless, I can't thank you enough. I think I'm going to stay inshore tomorrow. I never put them on the charger as I thought my motors were keeping them charged. I almost always have my motors on when I'm fishing as I like to drift reefs and rig hop. I'll get my service dept to put two new ones in. Again, thanks. Normally my gauges are reading about 14.2 volts, is that right?

[Glen E]

when you read your gauges with the engines off, you are reading the batt voltage

when you read your gauges with the motors running - you are not reading the batts, you are reading the charging system

a good read on batts:

http://www.windsun.com/Batteries/Battery_FAQ.htm

[Joe Dirt]

Thanks Glen! I saw on my charger it had the Bulk, Absorption, and Float readout on the front and it was in bulk mode when I plugged it in today.

[cobra427mnsi]

You didn't say if your boat was in the water or out of the water during the time it wasn't used. If in the water, even though the isolator switch was off, I think the automatic bilge pump would still drain power if there was any water to be pumped out of the bilge ie: a small leak.

[Joe Dirt]

Out of the water, it stay in dry storage. I went down this morning and the batteries appeared to be fully charged. I was able to crank both motors without any hesitation and the gauges were reading 13.2 volts, and 12.2 volts whith the motors off. It appears that they just needed a good charge.

[Glen E]

12.2 is still too low...when you flatten a non agm batt, you take 20% of the life outta it...

[sandy tows]

You never did say how old your batteries were
What does "fairly new" mean?

[Joe Dirt]

Two months old