Simple Question

Solar Expert Posts: 334 ✭✭✭
HI

A just want to charge a 12V 70Ah car battery (nearing end of its life) with one 12V solar panel.

I have read if i recall correctly, that for such a simple set up you dont need a charge controller--- is this correct? if so, why can you get away with it in this case?

cheers
Larry

• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Simple Question

i disagree that you won't need a controller for your pv as there is nothing otherwise to stop overcharging the battery unless the pv is so low in current that it would essentially float charge it, but with such a low current it could never actually be employed to charge the battery. if it is your intent to just float charge the battery then fine you may get away with it depending on the specs of the pv.
• Solar Expert Posts: 3,009 ✭✭✭✭
Re: Simple Question

The only other way would be to have a panel powerful enough to properly charge the battery, and for you to babysitting the process to determine when the battery is properly charged, then disconnect the panel so as not to ruin the battery. Like a special cake in the oven, you wouldn't be able to turn your back on it.
An already charged battery connected to a small panel just big enough to maintain a float charge is a different story.
• Solar Expert Posts: 334 ✭✭✭
Re: Simple Question

What would the float charge be for a 12V 70Ah battery- i guess it would be around 13.5V... but how many Amps?
Re: Simple Question

Generally, a 1% rated solar panel is considered to be a float charge (and low enough current to not need a charge controller).
• 70 AH * 0.01 = 0.7 amps
A panel rated for ~Vmp=15 volts (note, "float charging" panels designed to run without a charge controller can have Vmp below the ~17.5 volts minimum you see us use for larger solar PV systems):
• 0.7 amps * 15 volts Vmp = 10.5 watt panel (lower Vmp)
• 0.7 amps * 17.5 volts Vmp = 12.25 watt panel (higher Vmp)
Note, our host does recommend a 2.5 watt maximum solar panel per 100 AH (12 volt) battery bank if no charge controller is used.

You probably would use the smaller wattage panels for AGM type batteries (low leakage currents), and the higher current panel for flooded cell/traction/older flooded cell batteries (which have higher self discharge rates).

You can search Ebay for VW Solar Charger or VW Solar Panel and find a bunch that are rated for ~3.2 watts and 0.170 amps (170 mAmps)--some cheap, some way too expensive.

And our host has a variety of smaller solar panels (none are "cheap" as these lower power levels).

You can still purchase a "cheap" solar charge controller just to protect your expensive battery bank against overcharging:

SunGuard 4.5 amp solar charge controller

As always, keep track of the specific gravity/resting voltage of the battery bank (at least once per month for a flooded cell bank) to insure that everything is OK.

Note that batteries stored below ~77F will have lower self discharge/longer storage life; and batteries stored >77F will have higher self discharge/shorter storage life... The 10C / Rule of 2 works well here (18F lower/10C lower will give you ~2x the life and 1/2 the self discharge--And +18F will be 1/2 the life and 2x the self discharge).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Simple Question

imho it would be better for you to take all doubt out of the question and use a controller.

if it is the type that stays at a higher voltage and you know you'll just want to keep it on float long term then one can add a diode in series to get a v drop of about .5v-.75v. wiring a switch across the diode could short out the diode to allow full voltage operation of the controller again making the lower float voltage a good option. do note that if you try to force high current through the diode due to the battery being drawn from that the diode should be rated for the max current that the pv could output.

now if you have no idea of what i'm talking about for the diode creating a float voltage then don't do it as it takes a bit of knowledge of these things to implement the idea.
• Solar Expert Posts: 292 ✭✭
Re: Simple Question

Going without a CC is doable, but not really controllable. What kind of charging will you be doing? If you are just trying to keep it topped off it shouldn't be a problem. I bought a couple of blem batteries a while back, \$35, and the sit on a cart in the backyard with a 75w panel attached to them. The FIL uses them for a couple of his ham radios a couple of times a week. They will probably last a year or two more based on my past experiences doing this. They were new to start with though.
• Solar Expert Posts: 334 ✭✭✭
Re: Simple Question

HI

Thanks for the replies. I was just about to ask whether adding a Diode or Resistance in series would help reduce the charge .

Basically it's an old 12V 70Ah car battery, that we just want enough charge to run a drill for a few minutes when we are out working... to avoid having to bring a generator.

I was charging it yesterday with a Kycoera 120W.. opened the cover to allow hydrogen venting... bubbling nicely.. it was giving about 4A charge, which is ok for bulk i presume, but as you say, risks overcharging once bulk stage is finished.

Hence I was going to ask if putting a resistance or couple of diodes in series, would be a good idea... i presume a resistance of around 10 Ohms?? or are my calculations way out?
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Simple Question

that confirms it that you should have a controller, even though that battery may not last long anyway, i'm sure you'll get a replacement battery soon enough as you didn't get that pv for just that battery's last days for even small pvs are somewhat expensive. the diodes or resistors won't take the place of a good regulation circuit that a controller would have. bite the bullet and buy the controller as it is certainly cheaper than the cost of the pv itself and cheaper than going through batteries.
• Solar Expert Posts: 334 ✭✭✭
Re: Simple Question

We're just using a spare panel we have lying around at the moment... it's more of an experiment than a practical exercise.... we can also get used car batteries for free (often people/garages change them before time).. and if they have enough charge left for the very simple tasks we want, we have almost a free mobile energy system ... how much does the cheapest 12V charge controller cost?

It could of course all prove a complete waste of time, but you never know
• Solar Expert Posts: 2,362 ✭✭✭✭
Re: Simple Question

Just go to eBay.com and look for "solar panel charge controller" tons of cheap ones shipped direct from China. Not an endorsement by any stretch of the imagination.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Simple Question

also you will need a vast number of those car batteries available, even if they are free, as they don't last long in any configuration other than starting a car. overcharging the batteries will make them go even quicker.
• Solar Expert Posts: 630 ✭✭
Re: Simple Question

I started with bits and pieces and the folks here gave me great information (after some good-natured ribbing). I put those pieces together with a Chinese controller like what Solar Dave mentions.

\$30 (New, not used) through Amazon, which I trust far more than eBay. I still use it for keeping a float charge on a spare auto battery. For \$30 I got 10A/20A controller with 12/24V function, switchable Low Voltage Disconnect, pretty lights, and a silk-screened cover to explain what the lights mean. Have it for a year and no problems. PM me if you want the exact make and model.
• Solar Expert Posts: 334 ✭✭✭
Re: Simple Question

Thanks, I expect I'll get a charge controller, it's not exactly going to break the bank, and will ensure the system works as best it can... will be interesting to see how long these old batteries last for the tasks we require..
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Simple Question

you may find the cc could revamp (probably the wrong word to use) some of them to a small degree as car batteries do not get a full charge with present car alternator charging systems.