Can I safely charge/maintain this battery from my car's cigarette outlet?

[chuchu]

Hi Folks,
I'm looking at putting together a small backup power kit using a 35ah agm battery, a 30 watt solar panel, and a 10 amp solar charge controller. I understand the 30 watt panel might take a long time to recharge this battery, so my question is - when I'm on the road, can I simply connect this solar charge controller to my vehicle's cigarette lighter outlet (12v/120 watts max), to charge the battery as I'm driving? Do I need a higher capacity charge controller to do this?

Ideally I would like to have something that I can SAFELY leave plugged in all the time and then only deploy the solar panel when we're camped for a while.

Thanks

[dwh]

Which charge controller?

If it's a multi-stage charger, it will try to bulk the battery up to 14.4v, then float it for a while at 14.2 and then finally drop it to float stage - somewhere between 13.2v - 13.6v depending on the charge controller.

But your voltage regulator/alternator doesn't work that way. The voltage regulator turns on the alternator when the voltage drops to some point - usually 13.5v or so - and turns it off when the voltage reaches some point - usually 14.5v or so. The voltage of the system is always going up and down when the engine is running.

With the engine off, the truck's battery is going to settle down to a "resting" voltage after a while - usually 12.8v.

In other words, your truck's charging system is not going to supply a high enough voltage for long enough for the charge controller to do a 3-stage charge.


There is a reason why "12v" solar panels always put out over 15v.


Any non-MPPT charge controller just does one thing: It connects the solar panel straight through to the battery, and disconnects when the battery is full.

Fancier controllers, such as PWM or a multi-stage PWM will do that same exact thing, but they'll do it using a connect/disconnect pattern which does a better job of charging the battery. But if the incoming voltage isn't high enough, they can't bump it up - all they do is connect the input straight through to the output.

MPPT controllers can down-convert incoming voltage that is too high (that's how they do their magic - lower the voltage, amperage goes up), but they also don't bump up, so if the incoming voltage is too low to work with...

There are 3-stage DC-DC chargers, but they bump up the incoming voltage to make it high enough before feeding it out to the aux battery.


A simple on/off controller would work, but isn't required; you could just make a cable with a lighter plug on one end and some alligator clips on the other. It would do just as good a job.


[EDIT: OOPS. Forgot the last line...]

You could, but I wouldn't. Better to get a lighter to lighter jumper doohickey and put alligator clips on it to charge your little spare battery:

http://www.amazon.com/Wagan-EL9796-J.../dp/B00005OWK2

[chuchu]

Thanks for the detailed response dwh. The charge controller I'm looking at is this one, which appears to be a PWM type.

If I understand what you're saying, the charge controller might work, but since the truck does not output a steady 15+ volts like a solar panel would, the charge controller could have trouble fully charging the spare battery. If I were to make a simple patch cord to directly wire my spare battery to the lighter plug and leave it plugged in all the time, would there be A) any danger of over-charging the spare? B) any ill effects to the truck's starter battery or fuses? The lighter outlet is a 10amp fuse and only comes on when the ignition is on.

Also, I'm guessing the lighter-to-lighter doohicky that you posted essentially a direct patch cord with status lights in the middle... or does it employ more advanced logic?

[dwh]

At a quick glance, that charge controller looks to be one of the piece of crap Chinese controllers that are sold under various different names. This is not to say it won't work - it might work just fine. You can do what you want, but I personally would not buy a cheap "who knows what's really going on in there" controller.

Looking at the description on the Amazon page you linked, I see a few red flags...

First, it says it does an equalization charge to 14.8v. AGM batteries are NOT supposed to be EQ'd. It can be done, but most manufacturers specify a procedure of very precise voltage/amperage/time that basically requires a lab spec power supply instead of a battery charger. So if you can't disable the EQ function on that charger, don't use it for an AGM.

Then it says this: "4 Stages: equalization, PWM, Boost and Float". That's marketroid BS.
EQ is not really a stage...it's a maintenance function that's supposed to be done occasionally - like once a month for flooded batteries and never for AGMs unless there is a specific problem with the battery.
PWM is not a stage at all, it's a type of charge pulsing.
Boost and Float - some manufacturers use the word "boost" instead of the word "bulk". Bulk is an actual charge stage - basically feed the battery all it wants until it reaches 14.4v. Float is when the battery is charged and the charger drops the voltage to just keep the battery topped off. A 2-stage charger does a bulk stage and then drops to float.

So translating the marketroid speak into plain speak, it looks like a "2-stage PWM charger with some sort of EQ function".

If that really is what it is, it will probably do a decent job of battery charging. And if you can disable the EQ function, it probably won't toast your AGM.



As for your questions about charging the battery from the truck's system. ..

Answer to A) is: No danger of over-charging. The battery will get charged to the same voltage as the rest of the "12v bus" on the truck. I.e., the same voltage as the battery under the hood.
Answer to B) is: No ill effects to the truck at all.

The only real gotcha is drawing power from the aux battery and backfeeding it into the truck. It won't hurt anything, but replacing the blown lighter fuse every time the starter tries to suck all available power might get annoying after a while.

There are several different manufacturers of the lighter doohickey. I would assume that they have a diode to prevent the backfeed problem I just described, but I haven't done any real research into those devices.

[DaveInDenver]

If it's running an 'equalization' at 14.8V, then it's not equalization but bulk. Some manufacturers will give a process to equalize an AGM but most will not as it's very easy to ruin them doing that. Equalization for an AGM would be done at roughly 2.56V/cell, which is at least 15.4V and sometimes as high as 16V (and this, like all charge voltages, should be adjusted for temperature). This is held for some period, typically 3 hours. You should measure specific gravity to determine if equalization is done or not, although with an AGM this is not really possible, another reason they should not be equalized.

For all batteries the other 3 proper stages are bulk, absorption and float. They are typically around 2.46V/cell, 2.36V/cell and 2.26V/cell respectively. Bulk charge is constant current charge until voltage drops, absorption is constant voltage until current drops and float is better thought of as a safe static point more than a charge state. The battery is not really charging during float but it's also minimally discharging.

A simple 2-stage charger will often eliminate bulk, rather they run absorption and float to eliminate the need for a constant current supply. This is also because you must handle quite a bit of current to correctly do a bulk, no less than 20% of the capacity of the battery. For example, you should be able to source 20A during bulk charging if your battery's capacity is 100A-hr. I personally use a 45A charger for a pair of 92A-hr batteries in parallel. If the charger cannot do the current, then it's not a good idea to run the charge voltage up to 2.46V/cell as you will probably do more damage than good.

[chuchu]

Thanks guys, so it sounds like, instead of risking the health of the agm battery with the charge controller, I would be better to wire it up directly to the lighter outlet and let the vehicle's alternator/regulator take care of charging it. Since this outlet is switched off when the ignition is off, I should be able to leave it plugged in and not worry about draining my starter battery.

However, as dwh mentioned, back-feed could be a problem if the starter motor tries to draw from it and that would blow the dinky 10amp fuse on the lighter outlet. I'm contemplating two possible solutions -
1. Install a diode on the little "patch" cord going from the lighter outlet to the agm battery (or find a lighter outlet jumper cable with reverse flow protection). This would probably be easiest, but from what I have read, diodes will also reduce the voltage passing through them. How might this affect charging ability?

2. Install some type of relay or rewire the rear lighter outlet so it only comes on after the alternator is running. This should in theory prevent the starter motor from trying to draw from the agm battery, but probably would not completely eliminate the possibility of back-feed.
[EDIT]: this might not even be necessary as I believe the lighter outlet, as well as most accessories will momentarily turned off when the starting motor is cranking. I will have to verify this.

[DaveInDenver]

Per your edit, I would check that it does turn off during cranking. That would solve the starting draw although you might have to be careful that there isn't some load even with the key off that would drain the little AGM. Is this plug within your reach? The simple solution is to just plug the aux battery in after you start the car and unplug it when you exit the car.

Yes, a diode has a non-zero forward voltage drop. Rule of thumb is 0.7V, but it depends ultimately on the type you use. A relay is a good option, plus you could let it connect only when the engine is running and open during cranking and engine off conditions.

I would not assume the vehicle charging system is going to optimally charge your AGM, but it should suffice to put a base bulk/absorption and let the solar float it while you are away. I'd personally just use the patch cable and periodically run the AGM on a proper charger to condition it, by which I mean let it run a full 3 cycle smart charge once every month or two.

[dwh]

35ah is basically a wheelchair battery. You might consider a wheelchair battery charger. Powerstream sells some:

http://www.powerstream.com/DCC.htm

(Click where it says "State diagram showing the charge algorithm" to see something interesting. )

[DaveInDenver]

35ah is basically a wheelchair battery. You might consider a wheelchair battery charger. Powerstream sells some:

http://www.powerstream.com/DCC.htm

(Click where it says "State diagram showing the charge algorithm" to see something interesting. )

That looks like a good charger.

LOL! That state diagram is interesting, not unlike stuff I've had the chance upon which to work.

SABC_body_r1_c1.jpg

It's about squeezing every ounce of life and capacity from the system.

[chuchu]

I just confirmed the accessory outlets are off when cranking so that gives me confidence that I can just leave it directly-connected in the back seat. The fancy wheelchair chargers look nice but I feel like they would be overkill for this application.