charging 12 v battery

[fredfl]

I have a 240 amp hr golf cart battery that I use for backup emergency power with a 750 watt inverter. I've been thinking about getting a solar panel to charge the battery but don't know how much wattage I can use without using a charge controller. I don't intend to use the setup for extended periods only for backup power in case of outage. Any help would be appreciated.

[12vman]

I'll assume the golf cart battery at 12 volts.. ;)

I'd figure around 10-15 watts of panel with a blocking diode. This will only provide a "trickle" charge to the battery to keep it topped off understanding that the battery was fully charged when the panel was connected.. (and not to boil it to death without voltage regulation..)

"Charging" the battery with such a set up is another story..

Let's say that you use the battery/inverter set up untill the inverter shuts down with the built in low voltage safety circuit most of them have. They usually shut down when the battery drops to 10.5 volts under load..

Let's also figure that you used about 1/4 of the aHr. reserve of your 240 aHr. battery.. 60 aHr. would need to be replaced to the battery plus some normal internal losses (around 20-25%) to bring it back to full charge.

Using the above scenario of discharge and using a 15 watt panel without a charge controller..

A 15 watt panel will provide around 1.25 amps of charge on a good day in full sunlight. (properly mounted and angled) In a days time it's possible to collect 5-6 aHr. of power to put back into the battery. Now figure in the internal losses. (25%) Let's take the larger amount.. 6 aHr. You're going to lose 1.25 aHr. to these losses per day. Actual gain to the battery is around 4.75 aHr./day..

By using these figures, it would take 12-13 "good" days to fully recharge your battery @ 1/4 discharge with no loads connected! It would be possible to discharge your battery with an inverter in an hour or so, depending on what you have connected to it..

The problem with this set up is you'll only have 1 short shot of back up and it would take days to fully recharge the battery. If you add more panel without a charge controller, you could/would destroy the battery!

Planning your emergency needs is very important! If you only require a couple of small lights that will operate within the 4.5 aHr. discharge limit per day, the above system will be OK. I don't see that happening while using an inverter..
~Don

[fredfl]

Hi Don,
Yes it is a 12 v battery. If I understand you correctly the fully charged 240 amp hour battery will only supply 60 amp hr before the voltage drops to 10.5 volts. I was under the assumption that I could use a much larger portion of the 240 amp hr capacity before the voltage would drop. Do you know the math involved in the calculation? I may have to rethink this if I can only use 1/4 of the battery capacity. Thanks for your help.

[12vman]

It's the amount of current that you're pulling from the battery at a given moment. If the demand is high, the voltage will drop quicker. If the demand is very light, it is possible to get more aHr's from a battery before the voltage drops to the fully discharged voltage..

Info from here.. (http://www.batteries-faq.com/activekb/glossary.php)

(AH) - The unit of measure for a battery's electrical storage capacity, obtained by multiplying the current in amps by the time in hours of discharge. Example: A battery delivering 10 amps for 20 hours = 10 amps x 20 hours = 200 AH.
*For a totally, completely dead battery*

And here.. (http://www.batteryuniversity.com/partone-13.htm)

Open circuit voltage State-of-Charge in % * Figure 3: BCI standard for SoC estimation of a 12V flooded lead acid car battery.

Test the battery at room temperature. Allow 4-8 hour of rest after charge or discharge.
Courtesy of BCI
12.65V *100%
12.45V 75% *25% discharge level!!*
12.24V 50%
12.06V 25%
11.89V or less Discharged *Totally dead*

You should never discharge your battery less than the 20-25% level! The above voltages apply to flooded lead acid deep cycle cells as well..

It gets complicated.. I use the "1/4 of the capacity" theroy to be simple and safe. You need to know your loads and figure from there..

Let's say you are using your fully charged battery with a 10 amp load connected to it. You use this load for 6 hours. You have used 60 amp hours from the battery. This is about 1/4 of the total capacity. If you let the battery set.. with no loads connected to it for 4 to 8 hours and check the open circuit voltage, (no loads connected) the voltage should read real close to 12.45 volts. This means that you discharged your battery to the 25% level. It still has 75% left in it BUT you don't want to discharge the battery any lower than 25%. Taking it lower than 25% in cycles, over and over again, will mess the plates up and destroy the battery..

So, in review..
Using a larger aHr. capacity battery will give you a larger 25% total aHr. level to work with and to avoid draining the battery below the 25% limit. It gives you more head room to play in..

Your inverter has a circuit that will shut down the unit when the battery reaches 10.5 volts. This is a warning that your battery is getting low. If your demand (load) from the inverter is high, it will kick in earlier but who knows the exact level of discharge the inverter has taken the battery! It's probably already past the 25% "safe" level, so be careful..

Your inverter has a 10 to 1 ratio as for demand on the battery. (excluding inverter losses) To produce 1 amp of AC current, the inverter will draw 10 amps from the battery!

And don't forget.. Ya gotta replace the energy that you've used.. * ;)
~Don

*Edited to highlight important stuff..*

[fredfl]

Thanks Don. I think I am finally starting to understand this I looked at some DOD graphs and indeed the battery life triples with a 25% DOD vs 50% DOD. Since the setup I described initally is only for backup power in case of a power outage I think the setup will provide me with what I need in case of a limited outage. I also have the option of running off the car battery if I deplete the golf cart battery. But this brings up another interesting question. We use a 48 volt golf car as our primary transportation (we live in a golf cart community with all stores etc. accessable by golf cart). I had looked into the possibility of using a solar panel on the roof to charge the batteries and due to excessive cost rejected the idea. The thing I didn't consider was the possibility of decreasing the DOD and increasing the battery life. Right now a new set of batteries would cost $1000 or better and typical life is 3 years. The solar set up from a supplier is about $2000. I will have to think about how to calculate the feasability of this if anyone has any ideas I'd appreciate it. I'm guessing that a typical trip for us would cause a 30% DOD and every third trip probably 50% DOD.

p.s. I calculated monthly charging costs at about $6 and cost per mile of about 2 cents.

[12vman]

Quote..
"I'm guessing that a typical trip for us would cause a 30% DOD and every third trip probably 50% DOD."

How did you figure these amounts? Do you know the aHr. of the battery on the cart?

If you are adding current to the battery during the day, no matter how much, you are lowering your DOD somewhat. Charge would build up as you were shopping or doing other stuff while the cart is setting parked, provided that it's parked where the sun could hit it. Every little bit helps..

How much room do you have to install panels? Even if you can't totally charge the battery, it would help on the level of discharge. Instead of 3 years, you might get 4-5 years just by adding a little current to them each day. If you could pack 80-100 watts on the roof, that's a considerable charge!
~Don

[fredfl]

The commercial solar panel offered for golf carts is 180 watts. It replaces the existing roof of the cart. They advertise a 30% increase in range. I guessed at the DOD by measuring the time to recharge. On short trips it takes about 3 hours to fully charge, long trips up to 6 or 7. The batteries are (4) 12 volt 240 amp hour trojans. The motor is 3.3 hp. Another possibility would be to use a smaller panel and just trickle charge with a blocking diode. What do you think?

[12vman]

180 watt panel @ 24 volts= around 7.5 amps of charge

(4) 12 volt batteries @ 240 aHr. (series-paralleled)= 480 aHr.

3.3 hp. motor.. 750 watts per h.p.= 2475 watts

480 aHr. battery reserve= 11520 watts (total) 1/4= 2880 @ 25% DOD

You could race 10 speed bikes for about an hour and discharge your battery to the 20-25% DOD.. ;D

It would take 16+ charging hours for the 180 watt panel to fully recharge the battery.. :P (@ 20-25% DOD, 4 hr/day,4 days) *assuming my location*

What are the measurements of the roof? How much trickle charge would you like to add?

*The aHr. usage will vary depending on the speed/terrain conditions.*
~Don

[fredfl]

I'm thinking that the most economical thing to do would be to use the maximum size panel that would not require a charge controller. There are some complications that I didn't mention one being that the golf cart has a computer controller to monitor the charger and perhaps does other things that I am not aware of. I've talked with the service people and about the only thing I get from them is that I will void my warranty by installing the full blown pv system. Another complication is that the body is one piece so replacing the roof is not easy. The golf cart is a 48 volt system so I would need a 48 volt panel. I'm not sure how large a panel I could use without going to a charge controller or if anything this small is even practical. The roof size is about 3 ft by 4 ft. I'm really dissapointed that the manufacturer CLUB CAR does not offer their own setup as this would eliminate possible warranty problems. While I think that the solar panel at current costs is not economical many people would buy them for the novelty aspect. This cart costs $9000 3 years ago.

[12vman]

Ooops. My above calculations are all wrong. 48 volts.. I misread.. Gotta do it again.. LOL * ;)

Me thinks they are tryin' to scare ya! I can't see where a charge system outside of any of the electronics in the cart could mess with anything! Your connection would be right at the battery and I can't see where a charge controller would have any effect on anything. All a charge controller does is monitor the battery and adjust voltage/current from the panel. I think those guys need an education.. * ;D

"The commercial solar panel offered for golf carts is 180 watts."

"I'm really dissapointed that the manufacturer CLUB CAR does not offer their own setup as this would eliminate possible warranty problems."

Mebby Club Car needs to get on the stick.. 8)
~Don

*Edit Spellin'..*