Charging a battery

kau · January 2017

Is it okay to charge a 12 volt battery straight off a 70 watt panel putting off about 19 volts, if I monitor manually and remove before batt is over charged.

littleharbor2 · January 2017

If your panel has 36 cells it would be a 12 volt nominal panel. When connected to the battery you will notice the voltage output drop to somewhere below 14.4 volts depending on the state of charge of the battery. You can crudely charge your battery by monitoring the voltage but to do it properly you need a charge controller to do the absorb phase which holds the voltage steady at a predetermined voltage and allowing amperage to flow and complete the charge cycle . After absorb the controller will go into float and maintain your battery at 100% full until your battery loads pull it below full at which time the charging cycle begins again, provided there is sun, Ie; the next day.

oil pan 4 · January 2017

The short answer:
No.

mike95490 · January 2017

If you check every 15 minutes, log when the battery is at the proper absorb voltage, and disconnect before you cook the battery, it's easier to get a charge controller, unless this is a one time thing.

Marc Kurth · January 2017

kau said:

Is it okay to charge a 12 volt battery straight off a 70 watt panel putting off about 19 volts, if I monitor manually and remove before batt is over charged.

Depending upon the size of the battery (or batteries) and what/when the load profile looks like - you may have no risk of overcharging at all - or you can cook it rather quickly. It's like asking: "How long is a piece of string?" Well, it depends............

Generally, it is never a good plan to feed batteries uncontrolled voltages.

Marc

kau · January 2017

littleharbor2 said:

If your panel has 36 cells it would be a 12 volt nominal panel. When connected to the battery you will notice the voltage output drop to somewhere below 14.4 volts depending on the state of charge of the battery. You can crudely charge your battery by monitoring the voltage but to do it properly you need a charge controller to do the absorb phase which holds the voltage steady at a predetermined voltage and allowing amperage to flow and complete the charge cycle . After absorb the controller will go into float and maintain your battery at 100% full until your battery loads pull it below full at which time the charging cycle begins again, provided there is sun, Ie; the next day.
Thanks little, my cc croaked

littleharbor2 · January 2017

Kau, your welcome. Apparently the description wasn't necessary. Sounded from your opening post in the solar beginners corner you might not have done this before.

Estragon · January 2017

You will also want to keep an eye on temps as voltage reaches absorb. You might be able to do a sort of crude PWM absorb by connecting and disconnecting the panel.

PNjunction · January 2017

Heh, actually you will undercharge if you just pull the panel off the battery when it reaches say 14.4v and you have an eagle-eye on your voltmeter. However, If you leave it connected, the voltage of the battery will eventually try to reach the voltage of the panel, about 18v for a nominal 12v panel, and you will quickly kill it.

In a total pinch on a deserted island, this technique might be enough to start your vehicle, even though it is not fully charged.

If you use the eagle-eye voltmeter monitoring method, you'd have to emulate the old style ping-pong hysteresis controllers. That is, let the terminal voltage rise to say 14.4v, pull the panel, and then let it rest until the voltage of the battery drops to 13.2v. Re-attach the panel, and once again remove it when the voltage is 14.4v. Rinse and repeat hundreds of times. Each cycle of this gets quicker and quicker. Take a phone call, turn around, and you are cooking your battery with high voltage.

Even if you DO have a "hysteresis" type controller (such as cheap HF, Coleman, Sunforce 7a types), hysteresis charging is VERY inefficient and was common back in the 1970's.

In other words, unless you are on a deserted island with nothing but a panel, voltmeter, time on your hands, and a battery, get yourself a solar controller - pwm at least.

Estragon · January 2017

@PNjunction - that's pretty much the crude PWM I had in mind. Note OP said his cc "croaked". I assume he wasn't talking about a sound it made :-)

mvas · February 2017

kau said:

Is it okay to charge a 12 volt battery straight off a 70 watt panel putting off about 19 volts, if I monitor manually and remove before batt is over charged.

At what voltage, do you think, the battery would be 100% fully charged
when attached to the 70 Watt solar panel and in the full sun ?

It really depends upon the size of the battery, in Amp-Hours.

A typical 100 AH wet battery would reach 15 Volts @ 4.6 amps and will be about ~95% recharged.
The above describes a typical Bulk Charge.
At this point, we would normally switch to a constant 15 Volts ABSORB, to taper the amps until 100% recharged.
But that is not going to happen if you leave the battery connected to the Solar Panel.

So at this point, if you do leave the Solar Panel connected ...
You would only need about another 5 AH ( 100 AH battery x 5% remaining )
That means another ~1 hour (max) of charging at 4.6 amps.
That would push the final voltage near 16 volts !

I would disconnect the solar panel from the battery when either:
a) The battery reaches 16 volts @ 4.3 amps or
b) one hour has passed since the 15 volt reading
which ever occurs first.
The battery will be very near 100% recharged.

Voltages at 70° F.
Measuring the SG and reading no increase is the best way to know 100% recharged.

stranger · February 2017

How about a potentiometer in the circuit to give a specific voltage drop, to regulate the voltage after absorb voltage is achieved?

Okay, a high-power pot could be expensive. What other kind of variable load would be appropriate in a pinch?

Edit: I was thinking potentiometer, not variac

mcgivor · February 2017

I would get a cheap 10A charge controller <$15, and call it a day, there are better things to do with time, unless you're locked up in jail somwhere.

stranger · February 2017

> @mcgivor said:
> I would get a cheap 10A charge controller <$15, and call it a day, there are better things to do with time, unless you're locked up in jail somwhere.

Oh, no doubt. A charge controller is the way to go. I was trying to think outside of the box, MacGyver-style. There have been a couple of winter days that my battery bank was in dire need of a charge and there was no propane for the generator, because the weather kept me from driving the 8 miles of mud to get to town 20 miles away. Pop the hood. Hook up the jumper cables to the inverter to power the battery charger. Yeah, the battery charger was underpowered, but it got me through the day. I would have jumped it all the way to the battery, but my cables weren't long enough.

stranger · February 2017

Maybe we could make a charge controller out of coconuts and seashells. Eh, Professor?

kau · February 2017

Marc Kurth said:

kau said:

Is it okay to charge a 12 volt battery straight off a 70 watt panel putting off about 19 volts, if I monitor manually and remove before batt is over charged.

Depending upon the size of the battery (or batteries) and what/when the load profile looks like - you may have no risk of overcharging at all - or you can cook it rather quickly. It's like asking: "How long is a piece of string?" Well, it depends............

Generally, it is never a good plan to feed batteries uncontrolled voltages.

Marc

kau · February 2017

stranger said:

Maybe we could make a charge controller out of coconuts and seashells. Eh, Professor?

Thanks everyone for the feedback, I am getting use using this forum, seems to be set up quirky, anyhow, I decided to keep the battery up with a generator and a charger(quik charge at 15 amps), I know this isn't the best way. I use the battery once a week to run a 12volt pump to water gardens. I don't seem to be able to edit my post.:( Do other have trouble on this forum? As I was trying to say, I will be installing a charge controller for that set up. I would like to change the subject, I will start another thread. Thanks again!

BB. · February 2017

You should be able to edit your post up to 1 week after it has been posted... There should be a little black "gear" in the upper right of the box around your post... Click on that and you should see Edit/Delete.

If you need to change something later--Just send me a PM and I can take care of it.

-Bill "moderator" B.

westbranch · February 2017

and you have to click on that option too, "EDIT 1 week" is a bit of a misleading statement...

archsabino · February 2017

Hi Kau,

Thanks for this query. A panel with 36 cells means that it's a 12 volt nominal panel. If you connect the battery, your voltage output might drop. Well, sure you can charge it, but you're going to have to monitor the voltage. It'll have to go through the absorb phase wherein you'll need to have a charge controller.

Thanks,
Archsabino

Johann · February 2017

kau said:

stranger said:

Maybe we could make a charge controller out of coconuts and seashells. Eh, Professor?

Thanks everyone for the feedback, I am getting use using this forum, seems to be set up quirky, anyhow, I decided to keep the battery up with a generator and a charger(quik charge at 15 amps), I know this isn't the best way. I use the battery once a week to run a 12volt pump to water gardens. I don't seem to be able to edit my post.:( Do other have trouble on this forum? As I was trying to say, I will be installing a charge controller for that set up. I would like to change the subject, I will start another thread. Thanks again!

Could had used a 25 watt 12 volt automotive bulb in series with the panel to charge with about 2 amps or a automotive high-beam bulb to charge with about 4 amps. It could had charged the batteries slower to keep it alive, but there may be still a need to monitor and disconnect the battery.
But at the end, the amps needed to be used depends on the size of the battery size.

Charging a battery

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