advice on simple alternator excitation circuit?

here's my problem:

i have a 3 battery 160amp isolator under my hood such that my three batteries do not draw off each other (1 for car, 1 for amps, 1 for 'puter/etc)

the alternator has this wire coming off of it that reads the voltage of the battery and then decides how much voltage to spit out, called te exciter wire i suppose

it seems that when i connect a small wire from my main car battery to this wire, that battery becomes fully charged and causes the alternator to quit charging, but one of the other batteries may NOT be fully charged

so i suppose i need some circuit that will input 3 voltages, and output the lowest of those values out the other end... this sounds pretty simple, but the few EE classes i've had (and gotten c's in) arnt much help at this point (twas a while ago)

this will give that exciter wire the lowest voltage, and ensure all of my batteries are sufficiently charged

can anyone offer some design advice/insights?

Diodes?

look at op amps, something simple around some 741's should work. Problem I see here is that you could overcharge the already charged battery.
Get more complex and build one circuit with feedback into the alternator to keep it putting out say a constant 14v, then three separate 12v charging circuits powered from that which charge their batteries independently. I believe you can buy fancy charge controllers like that from RV shops for a $$.

I never thought about it really. Does this mean that the sure power isolator that I bought, will tell the alternator to quit charging when the main battery is full?

The surepower isolator doesn't tell the battery anything, the alternator stops charging when the regulator inside it or the vehicle engine computer tells it to.

Quote:

---

Originally Posted by n8scstm

I never thought about it really. Does this mean that the sure power isolator that I bought, will tell the alternator to quit charging when the main battery is full?

---

Well. . . the thing about putting batteries in parallel (you isolator should be doing this) is that the capacity of each battery is additive to a total capacity. When the first battery is getting charged, it should also be charging the other two batteries at the same time. If by chance one battery has a higher charge than the other the two batteries closest battery to it will should charge it. The first battery should give you an acurate voltage to give the "exciter wire". If your batteries are going dead, you may have hooked up the isolator wrong. .

how about some comparators, zeners, transistors, 3-input OR gate.

each battery feeds into a comparator input which compares that voltage to that of a 12V zener diode. If the battery voltage is greater than the 12V (From zener) the comparator is to output low - a logic 0 (this depends on the comparator u buy and how u set it up).

So, you'd have three outputs from comparators ....if one were to go below 12V that particular output would be logic 1 (high). These three are then fed into a 3 input OR gate. The OR gate will sense that if any battery is below the 12V it's ouput should be high (logic 1). This high value can be inverted so to trigger a PNP transistor to feed a reference 12V (say from another zener) into your alternator feed. The alternator will think the battery (any of them) is at 12V and start outputting it's charge.

Or you can be simplier as to take the comparator outputs into diodes then into the PNP transistor ....but I wouldn't do that. Hope what I said makes sense. It's the first thing that came to my mind so it might necessarily be the best.

The question is - does your isolator know where to put the charge? [I've never been a fan of isolators - the cheap ones usually just use schottky diodes that don't know what's going on...just simply isolate]

Quote:

---

Originally Posted by eugenen

The surepower isolator doesn't tell the battery anything, the alternator stops charging when the regulator inside it or the vehicle engine computer tells it to.

---

Then what is the sense wire and the acc. wire for? My understanding is that the acc. wire tells the isloator that the battery is full and then in turn tells the alternator to quit charging if the voltage is high enough. But this only reads voltage from the main battery. That's the whole point of this thread.

The acc or E wire controls the isolator circuitry, the sense wire is where the vehicle regulator gets it reference from, but yes it is a compromise since the second battery is isolated there isn't any way to know if its charged or not so you have the potential to under charge it. That is why you have the fancy charge controllers in the big RV's which have multiple batteries, they have a sense/regulation circuit for each.

Do it right with battery isolators. If the batteries are all connected in parallel, They will draw through the system and charge properly. The isolator is just for making sure that the main battery is only connected to the important systems of the car so you can be sure it will start up every time. I've had banks of batteries, they all charge properly. They all maintain almost the exact same level (variations in cell quality, I'll assume). BTW, batteries wont charge in order like that, they will all draw up to their capacity, without the first battery overcharging.

Also, you have 3 batteries under the hood? Methinks you are overdoing it. Your computer doesn't need much power, You didn't list the details of your stereo, so I'll assume it isn't huge. I only have 2 batteries, and that runs the car, computer, multiple monitors and a 5000 watt set of 4 amplifiers.

Dave