# how does a battery charger charge?

#### EricF

I've been having a nagging question for a while... I tried googling this and didn't get the answer. Let's say you have a dead battery and put it on a charger at say 10amps. The charger's amp draw needle swings over to 10. and the charging begins. As the battery is charging up,the amp draw lessens. With this, the charger is now pushing out less amps. So my question, does the battery draw the amps it needs dynamically, or does the charger somehow figure out what the battery needs and adjusts itself dymanically? (doesn't matter if it's a manual or automatic charger). I once had a completely dead battery, like 1 volt in it. I decided to waste time and put it on a 6amp charger. After 3 hours of the circuit breaker popping on and off, it finally stopped and battery was able to take a charge. THAT told me the battery was trying to draw more that the 6amps allotted (the charger was a 2/6amp) so the circuit breaker saw the attempt to overdraw 6amps and opened, the it closed a moment later, popped a few minutes after, and kept that cycle going. My guess till the battery had enough charge, it would be happy with the 6amps allowed. So me and a co-worker and going bonkers trying to figure this out. No real reason as to why we need to know this, just one of those.. "Hmmmm......." moments.

#### wrcsixeight

Healthy Depleted batteries generally can accept huge charging currents. At the 80% charged level the amount they can accept really takes a nose dive. Above this level higher voltages(electrical presure) would be needed to force the battery to accept more amperage. Auttomatic charging sources will limit voltages to 14.5 to 16 volts, generally and the battery will accept what it can at that voltage. manual charging sources have little or no voltage regulation, and can push a battery to too high a voltage, given enough time. Generally 16v is the maximum any flooded lead acid battery should ever be allowed to goto, and such a battery likely should be disconnected from the vehicle. 16 volts is 'Equalization' territory, which is an intentional overcharge to get all the cells to as high as possible specific gravity. It can be very hard on a battery, but it can restore a sulfated battery back upto its maximum available remaining capacity, which is much different than maximum available capacity when the battery was new. 16 volts for too long can cause thermal runaway. maximum battery temperature should be no higher than 120f. Charging sources should be removed when specific gravity, compensated for temperature increases, no longer rises. Do not apply EQ voltages to AGM batteries.

#### Quattro Pete

The charging profile will depend on the charger. Some smart chargers have a soft start period, for example, if the battery is quite dead. Here is the charging profile for my Battery Minder 2012, but it can only do 2 Amps max...

#### punisher

I think you have a grasp of it. A battery, in good shape (no shorted/sulfated cells) pulls amps inversely proportional to it's state of charge. Discharged battery pulls more amps than one fully charged. This rule generally works when dealing with old school fixed voltage chargers. Internal resistance also plays a role. New chargers charge at different voltages, pulse charging and everything in between. Not quite so many hard and fast rules about that.

#### eljefino

Originally Posted By: EricF
After 3 hours of the circuit breaker popping on and off, it finally stopped and battery was able to take a charge. THAT told me the battery was trying to draw more that the 6amps allotted (the charger was a 2/6amp) so the circuit breaker saw the attempt to overdraw 6amps and opened, the it closed a moment later, popped a few minutes after, and kept that cycle going. My guess till the battery had enough charge, it would be happy with the 6amps allowed.
I bet if you tried charging a half dead bank of bus batteries that breaker would have popped as well. Their resistance goes down when they get huge for any given state of charge. If you jump start a car with a dead battery, its alternator works real hard trying for a system voltage of around 14.2. This isn't great for the battery either as it's getting 50-ish amps crammed into it, but it also stabilizes the car's systems to get the battery at least a surface charge. I've jumped a dead car, driven it half a mile, turned it off, and had it catch again shortly thereafter-- with warm oil, gas in the intake manifold, surface charge, etc. The battery, for its part, needs that extra 10-15% of its discharge voltage to reverse the reaction and recharge. In between is a no-man's land. I had a battery with a shorted cell on some clunker I drove home; the 14 volts divided by five cells instead of six made those five boil and burp through their vents. When I stopped at traffic lights everything dipped to 10.5 volts, meaning slower wipers, dim headlights, slow blower fan, etc.

#### youdontwannaknow

Originally Posted By: EricF
So me and a co-worker and going bonkers trying to figure this out.
Let me try and attempt to answer your actual question. First of all its not about the charger but rather the voltage of the battery itself. More specifically the DIFFERENTIAL between the charger's voltage and the battery's voltage. Forget about fancy programmed chargers, even if you put the battery against a simple dc source like a power supply, or heck even another battery with higher voltage it will do the same thing you witnessed. Voltage flow works like water/fluid flow. It flows in the direction of lower resistance/pressure. If you connect a larger voltage source to a smaller one the current will flow in that direction and the amount of current depends on the differential which is what you are seeing. As the differential decreases as the battery picks up voltage the current it draws reduces. Hook up a 12V battery to a 12VDC source and you will barely see any current flowing. Hookup a 1.5V battery to the same source and it will draw so much current it might melt the battery.

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#### 440Magnum

Charging a battery from a fixed-voltage charger is like airing up a tire. When the tire is flat (battery dead) and you apply 32 PSI (13.8 volts), the air rushes in very fast (charge current is very high) limited by the size of the air hose (current limit on the charger). As the pressure in the tire (voltage of the battery) increases, the air flow (current into the battery) steadily decreases because there's less pressure differential (voltage differential) to drive the air flow (current). "Smart" chargers are different. They'll ramp up their voltage to force a constant-current charge period, then do a constant voltage final step, etc. to try to minimize stress on the battery and optimally charge it. Some of them will even switch in a resistive load after a few days to discharge the battery slightly, and then recharge it in an attempt to reduce sulfation without resorting to "equalization" charging. They're actually pretty good at it.

#### Smoky14

Look up a balanced bridge circuit, good info there.

#### 1 FMF

Originally Posted By: EricF
The charger's amp draw needle swings over to 10. and the charging begins. As the battery is charging up,the amp draw lessens. With this, the charger is now pushing out less amps. So my question, does the battery draw the amps it needs dynamically, or does the charger somehow figure out what the battery needs and adjusts itself dymanically? (doesn't matter if it's a manual or automatic charger).
what happens depends on the charger. most charges are both current limited and voltage limited. voltage limited should be obvious, the charger will never output more than 13.2'ish volts if it's specifically a float charger, or 14.5'ish volts otherwise. the amount of current the charger supplies depends on the rating of the charger... small ones that are a little block transformer that plug into a wall outlet are 1 amp... regardless of battery state of charge and resistance they will never put out more than 1 amp. the better chargers and more expensive ones control and monitor voltage and current output. the cheap ones always output just a fixed voltage, and then whether it's capable of limiting current or not depends on the design of the charger. the cheapest ones just rely on the battery's resistance to limit current, so for a dead battery which has zero resistance the battery will allow 60+ amps if the charger can supply it, so either the charger will have a fuse that will trip or the charger transformer will burn out and you wreck the charger. i have an old 2/10 amp charger i hacked up a while ago and put a rheostat in to control current. the internals of that only had the AC transformer that drops 120 vac to 14vac, then a simple rectifier circuit to give you DC. if i remember i'll post a pic.

#### 1 FMF

it was a while since i messed with that, in it's original form without my additions in the cover, the big thing in the middle is the transformer, want to say it drops ac voltage down from 120 to 16-17 volts. on the lower right where there's a red and blue wire coming off is the thermal fuse and that's in series with the red/black cables going out that connects to the battery, it's rated at 12v 20 amp. for a depleted battery that fuse will heat up and create resistance at 20+ amps so that is the current limiter. the 6/12 volt switch on the front selects between one of two taps off the transformer on the AC side. The two green wires on the right coming off transformer is what is stepped down to around 17 volts, and that is still AC. that silver bar on the back of the case is the rectifier which converts that ~17 vac to ~17vdc open circuit voltage. with a battery connected it tops out around 15 volts with the original front power switch turned on, the interesting thing i found out when i started hacking it is with the power switch off but the charger plugged in it would maintain around 13 volts on the battery, so it also acted like a float charger. and it's a straight 10 amp charger, there is no switch for low amp high amp charge, only a switch for 6 volt or 12 volt. so when charging a battery if you had the front switch on you had to really watch the battery voltage otherwise you would cook and ruin the battery. the hack job was a low cost attempt to be able to control output voltage via the rheostat i put into the cover.

#### y_p_w

Originally Posted By: 1 FMF
Originally Posted By: EricF
The charger's amp draw needle swings over to 10. and the charging begins. As the battery is charging up,the amp draw lessens. With this, the charger is now pushing out less amps. So my question, does the battery draw the amps it needs dynamically, or does the charger somehow figure out what the battery needs and adjusts itself dymanically? (doesn't matter if it's a manual or automatic charger).
what happens depends on the charger. most charges are both current limited and voltage limited. voltage limited should be obvious, the charger will never output more than 13.2'ish volts if it's specifically a float charger, or 14.5'ish volts otherwise. the amount of current the charger supplies depends on the rating of the charger... small ones that are a little block transformer that plug into a wall outlet are 1 amp... regardless of battery state of charge and resistance they will never put out more than 1 amp. the better chargers and more expensive ones control and monitor voltage and current output. the cheap ones always output just a fixed voltage, and then whether it's capable of limiting current or not depends on the design of the charger. the cheapest ones just rely on the battery's resistance to limit current, so for a dead battery which has zero resistance the battery will allow 60+ amps if the charger can supply it, so either the charger will have a fuse that will trip or the charger transformer will burn out and you wreck the charger. i have an old 2/10 amp charger i hacked up a while ago and put a rheostat in to control current. the internals of that only had the AC transformer that drops 120 vac to 14vac, then a simple rectifier circuit to give you DC. if i remember i'll post a pic.
The insides really depend on the technology used. A lot of the newer versions use switching power supplies and/or microprocessor controlled charging circuits. I've got everything from \$5 Harbor Freight maintainers (never plugged them in and I may be too scared to try for fear of starting a fire) to manual chargers, automatic maintainers, and a 3A/12A charger with automatic shutoff. Case in point is the Schumacher SEM-1652. Seems pretty basic. It's a 1.5A maintainer for the most part, but I have the current version and a previous version. The older one is heavier. I can tell there's probably a linear power supply in there. It doesn't say anything about reverse/short protection. There's a 6V/12V switch. It will probably charge up until it senses it's done. The newer one feels almost like there's nothing in there. It goes through seconds before the lights start up. I also connected it incorrectly once, and the reverse protection does work. There's no button or switch. I've also heard it has a 24 hour limit, where it will cut off charging if it senses that it's gone 24 hours and the battery isn't fully charged. The Harbor Freight ones (I got two slightly different ones) are made of an external power supply connected by wire to a control box (with one status LED), which then has the leads. It doesn't say, but I don't think it has more than .75A output, and that might be generous. The bigger one I have is kind of nice, but it seems weird since it's so light. You lift it up and expect that it should feel heavy like a battery, but it feels like there's the cord, cables, and a plastic shell. I kind of like the bells and whistles, like AGM/standard battery select, 3A/12A select (goes automatically to 3A maintain mode when done), and a voltage/percentage readout where it alternates between the two. Also - the display cuts out after a while, and pressing any button will trigger the display first, then after that they perform their listed function. It cost me about \$32 on Amazon, but their prices tend to be volatile. I think these newer ones have all sorts of features that simpler ones with only analog circuitry couldn't perform, like pulse charging and desulfation.

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