# Need a McGyver solution to charging a 12v battery

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#### petekw

##### New Member
Hey all! Great forum going here, glad I found it!

I need to charge a 12V battery with improvised means. The battery is lead-acid, sealed in gel, of the valve-regulated type. The official charger is (I think, from this ebay post) a 17V 1700 mA type. I need to charge it every now and then, and even a half-charge would be more than enough.

So my approach: I have an old transformer lying around from my electronics classes in college. I have tried to trickle charge it at 12V 300 mA but even after 12 hours I saw no improvement.

My transformer is rated at 5V 1000mA and 12V 300mA.

But I have found two leads that have a voltage difference of 18V. I dont know how much amperage will come off there...

My questions are:

1-- Is it safe for me to attempt to charge it at 18V? (5 minutes? 30 minutes? an hour?)

2-- How much amperage would I expect to be able to draw when using the leads that measure at 18V?

3-- Should I totally change my approach and just charge it off my car's battery, as if I was jumping another car?

I bought this electric mower for 20$, but it came with no charger. The previous owner had run leads to attach directly to the battery (the bolts coming through the top). See below: Our yard is quite small so I dont need a super professional approach, just one that works enough. Thanks!!!! all help is appreciated! #### mneary ##### New Member Your (+/-)12V should have 24V between them, capable of 300mA. To charge your battery, place this 24V in series with a 12V tail light bulb from the car (5 watt, not 27 watt). Monitor the voltage and don't let it exceed 13.8V. 30-40 hours if it's a totally flat 12AH battery, at 300mA. The lamp is NOT a charge monitor, it's just a resistor! It'll light, not at full brightness, and will stay about the same even after your battery is done. #### petekw ##### New Member Your (+/-)12V should have 24V between them, capable of 300mA. To charge your battery, place this 24V in series with a 12V tail light bulb from the car (5 watt, not 27 watt). Monitor the voltage and don't let it exceed 13.8V. 30-40 hours if it's a totally flat 12AH battery, at 300mA. The lamp is NOT a charge monitor, it's just a resistor! It'll light, not at full brightness, and will stay about the same even after your battery is done. I like the idea. Certainly in theme with my thread title Follow up: What is the typical resistance of the bulb you mentioned? I have some spare bits and pieces around - it's probably easier for me to pull some resistors off the shelf than remove a tail light bulb. Also, Why should I use the 24V option? There is indeed a set of leads that reads 24V, but I decided not to mention it because the 18V option was already exceeding the 17V original charger. #### unclejed613 ##### Well-Known Member Most Helpful Member the bulb works as a ballast, because it's resistance is nonlinear. it's resistance rises with temperature. with 24V across the bulb and battery, the bulb will "soak up" the difference between the 24V and the battery voltage. in doing so, it will also act as a current limiter. as the filament temperature rises, the resistance rises, until it reaches an equilibrium point and levels off. then as the battery charges, the current drops, the temperature drops and so the resistance drops, and the bulb maintains equilibrium. with the 24V source, you will want to monitor the battery voltage often. i have found it more accurate to actually disconnect the battery from the charger, and then measure the battery voltage. batteries with weak cells will measure much higher with the charger connected, because of increased internal resistance in the battery. i have a 10A open frame power supply i use occasionally for charging gel cell batteries. the supply has foldback limiting, which means if a preset current is exceeded, the regulator will start dropping the output voltage until the current is at the preset level. i find that by setting it to about 5 amps or so, the power supply will source the charging current and at the same time the regulator output will be folded back to the present battery voltage. as the battery voltage rises, it can be monitored with a meter. there is one problem, however. if the battery has a weak cell, the charging current won't trigger the limiter, and so the battery's state of charge can't be accurately monitored with the supply attached. a small 300mA charger with a lamp is going to behave similarly to my foldback limited supply, but if there is a weak cell it will read higher than the actual battery voltage with the charger connected. #### HiTech ##### Well-Known Member Just go get a small automotive battery charger... they are cheap and you can use it on your vehicle battery too. The variety that provide a switchable 2, 5, 10 amps are very common and affordable. #### petekw ##### New Member the bulb works as a ballast, because it's resistance is nonlinear. it's resistance rises with temperature. with 24V across the bulb and battery, the bulb will "soak up" the difference between the 24V and the battery voltage. in doing so, it will also act as a current limiter. as the filament temperature rises, the resistance rises, until it reaches an equilibrium point and levels off. then as the battery charges, the current drops, the temperature drops and so the resistance drops, and the bulb maintains equilibrium. with the 24V source, you will want to monitor the battery voltage often. i have found it more accurate to actually disconnect the battery from the charger, and then measure the battery voltage. batteries with weak cells will measure much higher with the charger connected, because of increased internal resistance in the battery. i have a 10A open frame power supply i use occasionally for charging gel cell batteries. the supply has foldback limiting, which means if a preset current is exceeded, the regulator will start dropping the output voltage until the current is at the preset level. i find that by setting it to about 5 amps or so, the power supply will source the charging current and at the same time the regulator output will be folded back to the present battery voltage. as the battery voltage rises, it can be monitored with a meter. there is one problem, however. if the battery has a weak cell, the charging current won't trigger the limiter, and so the battery's state of charge can't be accurately monitored with the supply attached. a small 300mA charger with a lamp is going to behave similarly to my foldback limited supply, but if there is a weak cell it will read higher than the actual battery voltage with the charger connected. Thanks for taking the time to educate me about the bulb's role. Good stuff. #### mneary ##### New Member I need to reiterate: If you're using a McGyver circuit that doesn't have charge monitoring, you need to monitor the voltage manually. And often. The bulb could be replaced by a resistor of 30-35 ohms 20-25 watts. Although P=V*I says <10 watts, it would burn any thing or person that gets too close (150 deg C is common). I suggested the bulb because most people don't keep a good assortment of 25 watt resistors, and a hot lamp is a nice visible reminder not to touch. I suggested 24V because I didn't really trust the "18V" that you happened to find. In retrospect, I could have guessed that you found 17V between the +5 and -12 lines. If you use the 17V, your resistor would be 10 to 12 ohms, 10 watts. Again, arithmetic might say <5 watts but a 5W resistor will be a burn hazard. Last edited: #### petekw ##### New Member I need to reiterate: If you're using a McGyver circuit that doesn't have charge monitoring, you need to monitor the voltage manually. And often. The bulb could be replaced by a resistor of 30-35 ohms 20-25 watts. Although P=V*I says <10 watts, it would burn any thing or person that gets too close (150 deg C is common). I suggested the bulb because most people don't keep a good assortment of 25 watt resistors, and a hot lamp is a nice visible reminder not to touch. I suggested 24V because I didn't really trust the "18V" that you happened to find. In retrospect, I could have guessed that you found 17V between the +5 and -12 lines. If you use the 17V, your resistor would be 10 to 12 ohms, 10 watts. Again, arithmetic might say <5 watts but a 5W resistor will be a burn hazard. Thanks!! I dont know how I measured 18V... You were right it -- I double checked it and I got 17.2V, which is exactly what I need (except this charger does not have too much current output...). I got these bulbs for 2$: two filaments, one @ 27Watts and the other at 8.3Watts. The inner lead's filament is like a straight thick wire, and measures about 2.2Ω I suspect this is the 27 watt filament because it is much thinker than the outer lead's filament - which is a thin coil and measures about .7Ω.

Please correct me if this assumption is wrong -- If I assume the output is limited to ~300mA, then :

P=V*I = 17.2 * .3 = 5.16W.

SO I could use either filament but just to be safe I will start by using the one which I guess is the 27 Watt filament.

Thanks again!

Also the cheapest battery car charger I could find in a store was like 40$. I like this cheap option better. #### petekw ##### New Member Seems to be working: I have it hooked up to the 17V pins. There is a dull glow from the bulb. ------------------------------------------- Time (min) ..........Volts measured on battery ------------------------------------------- 0 ................. 12.51 1 ................. 12.71 2 ................. 12.79 3 ................. 12.81 5 ................. 12.82 10 ................. 12.84 20 ................. 12.85 40 ................. 12.86 current was measured at 340 mA. I dont really know how long to push it... #### mneary ##### New Member Well, you're kind of confused, but all is working. Your power supply is probably not current limited, so you're using the bulb for limiting. You are only applying about 4V across the bulb, so you its power is only 4*0.3 or 1.2W. If you used the 8W filament, it would be not be bad for the filament, would reduce the strain on the power supply, but take longer to charge. How long to push it? Remember we said a flooded lead acid battery is full, and you must stop, once it reaches 13.8V. But if you have enough to mow the lawn feel free to stop any time before then. #### petekw ##### New Member Ok hmm I have measured it at at 340mA and after an hour of charging it was around 12.9 Volts and it seemed to have plenty of power when I started mowing. But then it was drained dead in like 3 minutes. So I only got 1/4 yard mowed. This time I'll charge it for like 4 hours. Thanks for all the advice. Last edited: #### giftiger_wunsch ##### New Member If you've only added about 340mAh of charge to an otherwise flat battery, it's no wonder it didn't last long trying to power a lawnmower. Charge for at least a few hours and try again. Last edited: #### unclejed613 ##### Well-Known Member Most Helpful Member if the battery has removeable vent caps, remove them while charging the battery, and put them back on after charging the battery. my brother once overcharged a car battery without removing the vent caps. the battery blew up literally in his face from the pressure. fortunately, it was cold very out and he was wearing heavy clothing and a ski mask, and he wasn't injured or burned. also, during and after the battery is charged, don't smoke near it, as the hydrogen that bubbles out of the battery is very flammable (and explosive if it happens to be in a 2:1 ratio mixture with oxygen). Last edited: #### mneary ##### New Member Didn't we say in an earlier message say that this tiny charger would take a couple of days? I didn't notice that you only charged for 40 minutes; my bad on that one. If it's flat right now, then loosen the vents like unclejed614 advised, and charge it for at least 24 hours (that's less than 8 Ampere Hours). If it's a 20AH battery, it'll take three days to fully charge. I don't recall reading how big your battery is; just keep an eye on the voltage.. Stop charging if it ever reaches 13.8. With only 340 mA charging current it's a slow proesss, but you wanted it cheap... #### Oznog ##### Active Member That's probably a half-U1 battery inside there. Those things are expensive for good ones, and you've gotta be very careful about how you charge them. Your plan of charging it "just enough to do the yard" will destroy them. Lead acid needs to be stored fully charged every time. But, leaving a "dumb" charger on too long will damage them in a different way. Even putting a trickle charger on them will sort of lead to the same problem, it'll take much longer to charge, but after a few days of charging you could easily forget and find weeks later that it's still on the charger. You need a real charger. Those batts, if they're still good, can easily be toast from bad maintenance including poor chargers. #### giftiger_wunsch ##### New Member if the battery has removeable vent caps, remove them while charging the battery, and put them back on after charging the battery. my brother once overcharged a car battery without removing the vent caps. the battery blew up literally in his face from the pressure. fortunately, it was cold very out and he was wearing heavy clothing and a ski mask, and he wasn't injured or burned. also, during and after the battery is charged, don't smoke near it, as the hydrogen that bubbles out of the battery is very flammable (and explosive if it happens to be in a 2:1 ratio mixture with oxygen). Wait wait let's get this straight: your brother blew something up while wearing heavy clothing and a ski mask and no one found that suspicious? Exploding car batteries... ouch. He's lucky he didn't get a face full of acid or a lung full of aerosolised acid; ski mask or otherwise, both would be unpleasant. #### unclejed613 ##### Well-Known Member Most Helpful Member a battery when stored in a discharged condition will deteriorate from sulfation of the plates. when the battery is charged again the lead sulfate will reduce to metallic lead dust, which will settle at the bottom of the battery and pile up between the plates and short them together. one this happens, you have a shorted cell (or more than 1) and the battery is useless. even if the cell doesn't short, the missing lead results in reduced capacity of the battery. #### #1supertech ##### New Member Pete - Read your electric mower batt related charging problem, and being a battery expert with over 33 years experience with all types - I have a few concerns as to your makeshift charger & charging scheme there. I also have some sound advice as to maintaining that$20 mower batt that sounds like it came with the mower - if that batt is indeed good to start with. My guess is that the batt is shot to start with, and it was an Ebay type buy? Was I right?

First of all - Are you sure it's a GEL cell and not an AGM type 12-volt VRLA batt? What brand name is it, and what is the Ah rating of the batt ? I'm just guessing here, but my guess is that it's either a single 17.2ah, or at the very least a 24ah AGM type batt with nut & bolt terminal lugs. I doubt it's a 33ah batt beings that it an older outdated 12-volt model that has long been replaced with 24 or 36-volt batt technology!

Secondly - Are you sure the batt is even any good? I'm guessing it's not unless it's an added newer batt that didn't come with that mower to begin with. I take it the mower works - right? If not then most likely the DC motor brushes are totally worn out and/or going bad, which is usually the reason people dump stuff on Ebay to begin with.

Forget that charger that you just had laying around there from your college electronics classes, and forget that suggested idea someone in post gave you as to hooking up a tail light as a ballast resistor of sort.

AGM & GEL VRLA type cells need precise CPU chip controlled chargers to properly charge them back up, and although people (so-called experts) say that GEL cell batts are more critical then AGM batts that is NOT the case! I've had GEL cells last me 10 to 15 years even if they were only used in underwater lighting systems.

My bigger AGM batts last me on average 9-10 years with proper charging, and with using proper CPU chip controlled fully automatic 4-stage chargers like the newer heavy-duty SEARS model 71225 charger that I now use day in and day out for all my charging needs. My smaller 1/2 amp fully automatic Float Maint chargers maintain all my AGM batts once they are topped off with the 71225 charger. No need to run the bigger charger all the time!

Though I think the 71225 is a bit pricey for you Pete at $80, there are much cheaper models out there that will suit your needs. Harbor Freight Tools has a few small footprint 12-volt fully automatic chargers for like$20-25 that will work just fine for your AGM batt there. Go online and check them out - otherwise visit an outlet store near you depending on where you live.

Their 1.5 Amp 12-volt Three Stage Onboard Battery Charger/Maintainer - HFT part # 99857 - is a fully automatic 3-stage charger that will especially work just great with AGM type batts, and it only cost \$20 + tax! It can be mounted inside the mower as well - IF there is enough room under the shroud cover? Just permanently connect it to the batt lugs - plug it in - and forget its there until you need the mower again.

I've personally tested this one out myself, and it works perfect with AGM batts! Switches roughly between 14 volts and 13.1 volts once it has fully topped off the AGM batt that's being charged up. Great little charger for the money!

CAUTION -

You should never constantly run any AGM batt down much below 75-80% of full capacity, as when you do then you greatly decrease the lifecycles of the AGM batt itself. You can abuse any batt, but it comes back to bite you 10-fold in the long run - in the form of out of pocket money spent to replace a premature damaged batt !

On the same token - both AGM & GEL cells absolutely need to be topped off every 4 to 6 months if they are to left in shelved storage mode (not being used at all). Such as with electric scooters, electric wheelchairs, trolling motors that use AGM and/or GEL cells, etc.

If AGM or GEL cell batts are left to drain down past a 6 month period - or longer - they can actually Sulfate totally, and then not be able to be de-sulfated at all. With GEL cells they can actually HOT SPOT the plates as a result of this same scenario - thus shortening their lifecycles drastically! I've opened up such GEL cells in the past and proved it - so I KNOW FOR FACT that it happens!

For instance -

Draining your regular FLA (flooded lead acid) car batt totally dry even 3 or 4 times will totally kill the batt! Bet you didn't know that?

With GEL cells the same applies, as if you totally drain one and go to recharge it - even with a good CPU chip controlled charger - IT WILL HOT SPOT.

I never knowingly take my AGM batts down much below 80% and 12.6 volts measured under load. If constantly using an AGM batt on any "DC to AC" Inverter you will risk lower batt lifecycles as well, as the cut-out (shutdown) voltage on them is factory preset very close to 10 volts on average, which is way too low for any AGM batt, much less a GEL cell. The Low Batt Inverter Alarm is usually factory preset at 10.6 volts, which again is way too low as far as AGM batt lifecycles are concerned. You're better off using a cheaper FLA Deep-cycle type batt if you intend to constantly use that Inverter in that manner. Deep-cycle batts are designed to take that constant punishment, and are way cheaper to replace as well !

Hope this sheds some intelligent light on your situation there, as I can only make logical suggestions, as well as show/state proven solutions to batt related problems. After that it's a popular Forrest Gump saying that usually kicks in.

I'm sure we all know what it is I'm hinting at as well.

Best regards,

Frank

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