Time to Charge Batteries?

[Fender]

I've had a 2A trickle charge on my brand new dual 6V batteries for a few days now and they are still charging.
How long should it take to fully charge the batteries? My charger is supposed to shut off once batteries are fully charged.

[alanmalk]

Help us by providing the following information: The amp hour capacity of the batteries.

Once we know that number then it becomes a matter of dividing the battery capacity by the charge rate. So, for 100 AH batteries, 100 / 2 = 50 hours.

But that is only valid for ideal situations. Charging at a 2 A rate is rather slow and may not trigger the automatic shut-off. There is always some lost energy and self-discharge so the trigger value may never be reached.

In that case you can just disconnect the charger after 50 hours (2 - 3 days would be fine).

[Fender]

Originally Posted by alanmalk

Help us by providing the following information: The amp hour capacity of the batteries.

Once we know that number then it becomes a matter of dividing the battery capacity by the charge rate. So, for 100 AH batteries, 100 / 2 = 50 hours.

But that is only valid for ideal situations. Charging at a 2 A rate is rather slow and may not trigger the automatic shut-off. There is always some lost energy and self-discharge so the trigger value may never be reached.

In that case you can just disconnect the charger after 50 hours (2 - 3 days would be fine).

The batteries are Interstate 6 Volt both labeled "225ah@20 hours".
I measure 13.14 V across the batteries.

[Iowa Dave]

They might take a little more than that but not much. 13.4 would be the top I think. I have a 2 amp trickle charger I use every once in a while. On a small run down garden tractor battery, it can take 24 hours to turn green and shut down. And that’s only a 35 or 45 amp hour battery. I know the battery is down but not dead when the mower will start but the blade clutch will not engage. Usually I have accidentally bumped the lights rocker switch to on in the daytime.
Iowa Dave

[alanmalk]

Originally Posted by Fender

The batteries are Interstate 6 Volt both labeled "225ah@20 hours".
I measure 13.14 V across the batteries.

More variables to confirm: My assumption is that your charger is 12V (nominal). (Or in other words you are not using a 6V charger on one battery at a time.)

Another variable - how full/empty are the batteries to begin with. You said "new", so partially charged at the beginning is a reasonable assumption. If you want a reasonable estimate of the current state of charge the procedure is to disconnect all charging and discharging and let the batteries "rest" for a few hours. Then use an accurate volt meter and compare the reading to a standard table (easy to find on google).

An example: If you determine the resting voltage to be 12.10 then the batteries are 1/2 full, they need 112.5 Amps for full charge. At 112.5/2 you get 56.2 hours. Call it 3 days to round up a bit and not worry about decimal points and a few other annoying variables (like temperature).

[Fender]

Originally Posted by alanmalk

More variables to confirm: My assumption is that your charger is 12V (nominal). (Or in other words you are not using a 6V charger on one battery at a time.)

Another variable - how full/empty are the batteries to begin with. You said "new", so partially charged at the beginning is a reasonable assumption. If you want a reasonable estimate of the current state of charge the procedure is to disconnect all charging and discharging and let the batteries "rest" for a few hours. Then use an accurate volt meter and compare the reading to a standard table (easy to find on google).

An example: If you determine the resting voltage to be 12.10 then the batteries are 1/2 full, they need 112.5 Amps for full charge. At 112.5/2 you get 56.2 hours. Call it 3 days to round up a bit and not worry about decimal points and a few other annoying variables (like temperature).

Charger is 12 volt and I'm charging both batteries at the same time.
When the trailer was delivered I measured the voltage and it was around 12.5 Volts. The trailer had likely been sitting with the batteries disconnected for a couple of weeks.
My charger can be switched to 4 or 6 amps which might work better. For some reason I always thought the trickle charges better for the battery.

[Lanark Camper]

Could you plug in the trailer and let the converter charge the batteries ?
Do you have solar on the trailer ? If not the converter will work to keep the batteries topped up.
Bob

[Fender]

Originally Posted by Lanark Camper

Could you plug in the trailer and let the converter charge the batteries ?
Do you have solar on the trailer ? If not the converter will work to keep the batteries topped up.
Bob

Could plug in. I don't have solar and I store the trailer inside so that wouldn't help anyway.
All is good, just wondering how long it usually takes to charge things up.

[John in Santa Cruz]

golf cart batteries should be bulk charged with at least 6 and as much as 50 amps. a 2 amp battery tender will just keep them charged, it would take months to put a charge on them. Frankly, if you can, just plug in the trailer, and let the power converter take care of them, they'll be charged by tomorrow morning.

12.5V at rest is probably around 75% charged already.

[alanmalk]

Originally Posted by Fender

Could plug in. ...
All is good, just wondering how long it usually takes to charge things up.

A rough rule of thumb - usually good enough for camping - is <Needed amp hours> / <Supplied amps>

So 112.5 amp-hours / 50 amps = 2 and a bit hours.

It is possible to go nuts by taking into account self-discharge, temperature, age of batteries, wire losses, phase of moon, etc.

6 volt batteries are rugged and not likely to be damaged by any current that the built-in charger can supply. They can be damaged by a lot of things besides large currents. A topic for another day (or a good deal of reading if you want to be a good parent to your batteries.)

[Iowa Dave]

We don’t use a lot of battery power. We often camp where we will get some sun all day or full sun for some part of the day. We have a 190 panel. I usually don’t look at the monitor the first thing in the morning but often look at it between about 10 AM and Noon. Very rare that it does not say 14.2 to 14.4 anytime after 10AM. If it’s shady site or raining it’s closer to noon.

We just don’t use a lot of battery power. Probably cause we don’t have an inverter, enjoy using the propane stove or Weber grill and bake something we eat
for a couple days when we have shore power and know we won’t in the ensuing days. Except a toaster oven pie. They don’t last very long.
Iowa Dave

[John in Santa Cruz]

Originally Posted by alanmalk

A rough rule of thumb - usually good enough for camping - is <Needed amp hours> / <Supplied amps>

So 112.5 amp-hours / 50 amps = 2 and a bit hours.

lead acid batteries are actually kinda tricky, when they get close to 80% charged, the current goes way down, and it takes about half as long as that 80% took to put the last 20% in them, this last 4 hours or so is called the 'absorption phase' (the first 80% is the bulk phase). Lithium Iron Phosphate batteries are a lot more linear, they will take the full charge current right up til they are 99% charged.

Also, all batteries take 10% or 20% more power to charge than they give back.

[KirkB]

Since Fender does not have solar, here's a summary of daily solar charging on my old 2014 ETI Panel and 2x 6V Interstates -

1. Bulk charge (max amps) to ~14.4V for one hour. On my 85W panel, 3 to 5 amps in bright sunshine. It may take some time to reach the 14.4V if more than minimal 12V usage the previous night. Depending on sunlight (or cloud/tree cover), this cycle might not complete on some days.

2. Trickle charge at ~13.6V - after the bulk charge, usually it takes about 0.5A (or less) to maintain the batteries at 13.6V. Of course, the amperage will vary if there is any load on the batteries (e.g. fans, charging devices, etc ).

If you measure the battery voltage while charging, and it is no more than 13-14 V, you are still topping up your batteries. If you can, just plug your trailer in to shore power, and charge until you are at 13.6V. The controller will keep your batteries charged at this level.