Charging a car or lawn tractor battery off of my setup

[max2k]

We all know that; I designed and built such circuits in the 60s. Has nothing to do with SunEagles statement. Bruce Roe

How come? I thought it directly contradicts his statement- one can take 12V battery and using this idea charge anything from 3V all the way to 50V and everything in between including another 12V battery. Again, this is pure theoretical point and there might not be commercial product available doing this.

[SunEagle]

How come? I thought it directly contradicts his statement- one can take 12V battery and using this idea charge anything from 3V all the way to 50V and everything in between including another 12V battery. Again, this is pure theoretical point and there might not be commercial product available doing this.

Well basically the OP just wants to charge a car or tractor battery. A 12volt battery by itself will not charge a discharged 12volt battery enough to get it to work.

Sure if you want to add a device to increase the charging voltage you might put enough into the "dead" battery to get a "start". But just hooking up a good 12v bat to a bad 12v bat will not work.

So plain and simple without adding any device to increase the voltage my statement that 12v will not charge a 12v battery is correct. But you can choose to not believe it if you want.

[Sunking]

I respectfully disagree- it sounds like OP just wants to get car started. That process takes much less charge: 100A x 30sec cranking about 1 Ah. He doesn't need to get battery fully charged- just enough to start the engine and then car charging system would take over.

I agree you do not have to fully charge the battery, but your theory is seriously flawed. If all it took was a 1 AH of 12 volt battery capacity, you could use "D" sized alkaline cells used in flashlights or 4 cell phones. Not going to happen. A deeply discharge lead acid battery internal resistance rises as it discharges, thus preventing it from delivering high currents. You have to get the SOC high enough to lower the Ri to acceptable levels.

I know a great way to charge a 12 volt battery with another 12 volt battery. I can use a 6 volt battery to charge a 12 volt battery. I can do it at any time. Piece of cake and extremely safe to do so. RC Hobby chargers can do that with one hand tied behind their back, standing on its head under water. No problem. RC Hobby chargers can charge any battery type of today and tomorrow from any DC Power Source like a deep cycle battery. In fact many RC pilots use a 12 volt optima or pair of golf cart batteries to charge their LiPo's in the field. Some of those LiPo's are 22.2 volts @ 10 AH.

He could also charge them with his 12 volt Deep Cycle batteries with jumper cables if certain precautions are taken. Many Battery Isolators for RV's have that function built into them for just such emergencies to charge the SLI battery with House battery. It is a simple bypass switch with a current limiter.

Back to Hobby Chargers. Some are bidirectional, they can pass the charge both ways.

[max2k]

I agree you do not have to fully charge the battery, but your theory is seriously flawed. If all it took was a 1 AH of 12 volt battery capacity, you could use "D" sized alkaline cells used in flashlights or 4 cell phones. Not going to happen. A deeply discharge lead acid battery internal resistance rises as it discharges, thus preventing it from delivering high currents. You have to get the SOC high enough to lower the Ri to acceptable levels.

I know a great way to charge a 12 volt battery with another 12 volt battery. I can use a 6 volt battery to charge a 12 volt battery. I can do it at any time. Piece of cake and extremely safe to do so. RC Hobby chargers can do that with one hand tied behind their back, standing on its head under water. No problem. RC Hobby chargers can charge any battery type of today and tomorrow from any DC Power Source like a deep cycle battery. In fact many RC pilots use a 12 volt optima or pair of golf cart batteries to charge their LiPo's in the field. Some of those LiPo's are 22.2 volts @ 10 AH.

He could also charge them with his 12 volt Deep Cycle batteries with jumper cables if certain precautions are taken. Many Battery Isolators for RV's have that function built into them for just such emergencies to charge the SLI battery with House battery. It is a simple bypass switch with a current limiter.

Back to Hobby Chargers. Some are bidirectional, they can pass the charge both ways.

Now that is the ultimate answer, thank you- I had a feeling modern technology must be able to do better. I didn't know lead acid batteries take significant charge to reduce its output resistance, thanks for correction. I admit last time I charged one 'manually' (monitoring current/voltage/density and R_i at the end of the cycle) was somewhat 20+ years ago and at that time I was not interested in its behavior at the beginning of the charge cycle so it's all news to me .

[Sunking]

Now that is the ultimate answer, thank you- I had a feeling modern technology must be able to do better..

You are welcome. Hobby chargers have been out for some time now. They just need a source of DC Power. I have all kinds of high current DC Power Supplies being a Ham Radio operator. Designed and built most of them. I have a 24 volt 50 amp one I use on my PL8 pictured above. I can charge a dead Car Battery in an hour with 60 amps.

You are right, you do not have to fully charge a batter, but one thing you overlooked is it takes a great deal more than 100 amps. Most car batteries deliver 500 to 800 cranking amps. When you buy a battery, good ones list CCA (Cold Cranking Amps) and CA (Cranking Amps) Basically both mean the same thing, but specified at different temperatures. CCA is specified at 0 degree F at 1.2 volts per cell (7.2 for a 12 volt battery), and CA is specified at 32 degrees.

From that you can calculate Internal Resistance Ri = 5.4 / Amps

[Basketcase]

Y'all are way smarter than me. I'm just trying to fire up an old dodge or mow my lawn. lol. So the above "hobby charger" could charge off of my main battery bank and bring the lawnmower battery up to proper voltage then? (or truck battery) If so, that's what I need.

[SunEagle]

Y'all are way smarter than me. I'm just trying to fire up an old dodge or mow my lawn. lol. So the above "hobby charger" could charge off of my main battery bank and bring the lawnmower battery up to proper voltage then? (or truck battery) If so, that's what I need.

There are a number of hobby chargers out there that can work on different battery chemistries. You just have to make sure what you want will charge the battery chemistry you have.

I own one similar to what Sunking posted called the Cell Pro Multi 4 made by the same company Revolectrix. It is less powerful at 4 amps but still can handle all types of batteries including Pb (lead), LiPo, LiFe and NiCd.

[PNjunction]

Probably the simplest option for doing what you want sporadically is a Tecmate-Optimate model TM500 dc-dc charger:

https://tecmate.com/products/tm500/

Keeps track of the source as well as the target battery's health. At only 2A, it would not be recommended for deep daily discharges, but for your attempts to revive deeply discharged batteries (provided they haven't been that way for so long that they have suffered major internal damage / hard sulfation expansion and the like), this would be ideal.

LED's give you the status of what's going on, and if a Tecmate trips a failure indicator, you can basically trust it to tell you to stop wasting your time and recycle the knackered battery.