# charging battery directly from a source, what could go wrong?

I have a 100A lead acid deep cycle battery and also have a car's 12V 100A charger (alternator with AVR)

At 1500RPM the alternator will produce nearly 100A.. and the battery is directly connected to the battery, will the 100A be pushed into the battery? Or will the battery act as a load and take only what it needs? And how can I calculate the battery's resistance? I don't have the datasheet nor know the brand (unbranded).

I have been searching on the internet and they all just talk about the relation of the batteries resistance to its capacity.. nothing about how to measure it nor about the charging capabilities... (Can I get a reference on how to measure it please?)

So, is it safe to connect it directly? (The built-in AVR cuts of when fully charged) or do I need a limiter in between?

• If not included you need a beefy diode to prevent the battery to discharge through the inverter, apart from that it's safe to connect all together. – Vladimir Cravero Jun 12 '15 at 9:17

Okay, lots of stuff to talk about.

The thing about battery charging is that the battery has its own voltage, so resistance is only half of the story. If the charger is at 12 volts, and the battery is at 11.6, then the .4 volts difference is all that is pushing the current into the battery. This small voltage divided by whatever resistance it finds in the cables and battery determines that current. You might see a 100 amp peak, say when starting an actual car. That causes a rather large voltage transient.

And then, that isn't exactly right either. The voltages that you see while charging will be really very close to each other, and what's actually happening is you are replacing electrons in the battery chemistry to make its reaction go backwards. Its just a lot easier to model this mathematically as an ideal voltage source behind some internal resistance.

But the resistance is a real thing. Professional tools actually place a small value of AC current through the cell and take an AC voltage reading across the cell. This and Ohm's law gets you the value of the internal resistance for the pack or cell, wherever you can get terminals to check it. But, generally, this won't matter to you.

Hooking it all up together will work, with the caveat that diode Vladimir mentions needs to be in the line so the battery doesn't discharge through it. It may be built into the alternator already, maybe not. Do check that. What will happen is that when the battery voltage and the alternator voltage are the same, with no other loads connected, no current will flow.

• So is the difference in voltage that drives the current and not the actual source voltage... I failed to think of that! That makes a lot of sense! Then if you get a 30A charger and hook it up to a battery with the main being the source the battery will only get what it needs (ohms law) and not 30A? Also, is it safe to charge it with a constant 16V rather than 2V higher? And based on the difference in voltage and the battery taking in many amps it means that most batteries' resistance is usualy under 0.5ohms right? Sorry for asking a lot in a comment – bakriawad Jun 12 '15 at 11:11
• 16 volts would continually gas the battery. That's a chemistry problem. An actual charger has a programmed charge rate that is controlled to maximize battery life. And it has been far too long for me to say what a typical internal resistance is. The 30 amp charger would as you say only deliver what is needed, so to speak. – Sean Boddy Jun 12 '15 at 18:02
• Thank you very much! Learnt some things I been searching for from 3 weeks ago in under 2 minutes! Really appreciate it – bakriawad Jun 12 '15 at 21:20

A modern car alternator contains a 3-phase AC generator, a full wave bridge (6 rectifiers) and a voltage regulator (older ones had the voltage regulator external). Note to electronics folks (in case it's not obvious): AVR in this context means Automatic Voltage Regulator not the chip from Atmel.

It's designed to be connected directly to a '12V' lead-acid battery. The voltage regulator will attempt (by changing the alternator field coil voltage) to maintain a constant ~14V or so output voltage. If the battery is discharged it will draw current up to what the alternator can supply, but when it approaches full charge the current drops to a low float charge current. That's a characteristic of the battery rather than anything the alternator does- it just tries to maintain a constant output voltage regardless of what you hook it up to. The voltage regulator output voltage is 'tuned' to the chemistry of a lead acid battery, and the current capability is not high enough to cause damage to a full size automotive battery. If you hooked it up to a small SLA battery it could definitely cause problems.

Note: That's the configuration used on simple cars- some cars such as one of mine require service software settings to adjust to the exact type of battery (AGM is different from conventional) and the ampere-hour capacity in order to optimize the battery usage.

• Thanks a lot, very helpful! I knew that it had 6 poles but I never knew it was 3-phase! That's awesome!! – bakriawad Jun 13 '15 at 9:37