Battery Isolator with Battery Drain

[OceanPlanet]

Originally Posted by Don Lucas

I don't understand the sense in having more than 1 house bank connected to different terminals of the battery isolator. Unless there is a way to isolate each bank, and where doing so connecting the alternator sensor to the bank still on service for the regulator.

My boat has 2 house banks and a start battery. Each bank can supply the main DC panel alone or by together. Also each house bank can be selected to the inverter.

All my charging, either engine or with the battery charger goes though the inverter switch to the house banks. So whichever bank is selected to the inverter (1,2,or both) gets charged. Normally I have both banks on service to both the main DC panel and to the inverter (which is normally off unless I need AC for something).

So on my isolator (just got a new one that doesn't use diodes with the big voltage drop) I have the engine alternator output and the house battery charge line (to the inverter switch) connected together on the isolator input. And 1 of the 2 outlet terminals connected to the start battery. So the alternator always senses the house bank, either 1,2 or both and the regulator charges based on the house bank on line.

Hmmm...this just might work. a couple things come to mind:

1) Are both house banks in the same or similar locations, temperature-wise? You need to decide which lucky battery gets the alt. reg. battery temp sense. If they are close to each other then probably doesn't matter.

2) The better alt regs have both a pos. and a neg. battery (usually a neg. alt output) sense. You will also need to decide where to put that. If a neg. alt output then of course on the alt. Just mentioning it.

Let us know how it goes!

[sailorboy1]

Originally Posted by OceanPlanet

Hmmm...this just might work. a couple things come to mind:

1) Are both house banks in the same or similar locations, temperature-wise? You need to decide which lucky battery gets the alt. reg. battery temp sense. If they are close to each other then probably doesn't matter.

2) The better alt regs have both a pos. and a neg. battery (usually a neg. alt output) sense. You will also need to decide where to put that. If a neg. alt output then of course on the alt. Just mentioning it.

Let us know how it goes!

Of course it works, it's been that way for 11 years. And it came from the factory this way and I bet they did it the same on lots of models other than mine. So lets say a 1000+ boats.

[nigelmercier]

Originally Posted by cal40john

Even though you can think of this as an effective short in one way, it is only true as long as both diodes are forward biased. There is no way current can flow from one battery to the other. For the current to leave one battery it has to reverse bias its diode at which point the circuit is open.

I'm sorry, but this is not true. I agree that this is not intuitive, so perhaps a little more detail will clarify:

Both diodes are forward biased, by the charging voltage. Being forward biased, their resistance falls, allowing some current from the higher voltage battery to flow back round the loop to the other battery: subtracting itself from the incoming current to the higher voltage battery, and adding to the current to the lower. Thus current flows from one battery to the other. Both batteries will read the same voltage as they are connected by the still forward biased diodes, so it does not matter which one has the sense wire.

I suggest any sceptics do an experiment, you will see that what I have said is correct.

[nigelmercier]

Originally Posted by donradcliffe

Nice try Nigel, but you'll have to do better than that if you want to lecture at my university. Each diode/battery connection is a separate circuit in the isolator, and there is no way you are going to get any significant current through a reverse biased diode (you are a long way from breakdown voltage), and there is no way you are going to discharge a battery through it.

Then I'm glad I'm not applying for a position there

This is nothing to do with breakdown, avalanche or any similar effect. The diodes are forward biased by the charging current, see above.

[bill good]

I will stick my neck out & expand on the use of two diode isolators just maybe to help the person trying to get his boat working a bit better. We still don't have a clear indication which units he has BUT I assume they are simple diode types. This would explain why the house batteries are draining the starting batteries IF NO RELAY OR SWITCH is used to isolate the starting battery. I would recommend the better way to use the isolator in this case is by using both isolators to prevent back flow. Each isolator be feed from each engine driven alternator. One leg of the each isolator feed the independent starting battery. If a battery switch is on board the other leg of each isolator is wired to it & if no battery switch then join these legs together to the house bank. There maybe a voltage drop across the diodes .2-.6 of a volt depending on the diode type but I would not be worried so long as min 13.8v is achieved.
KISS


Regards Bill Goodward

[DeepFrz]

Bill, the reason they are called battery isolators is because they isolate the batteries from each other when there is no charging voltage. Therefore a load on one bank/battery will not drain the other bank/battery. No need for 2 isolators.

[OceanPlanet]

Originally Posted by Don Lucas

Of course it works, it's been that way for 11 years. And it came from the factory this way and I bet they did it the same on lots of models other than mine. So lets say a 1000+ boats.

Hurray. Goes to show that your point about no need for the two output leads on the isolator. I just don't use them at all.

[OceanPlanet]

Originally Posted by nigelmercier

I'm sorry, but this is not true. I agree that this is not intuitive, so perhaps a little more detail will clarify:

Both diodes are forward biased, by the charging voltage. Being forward biased, their resistance falls, allowing some current from the higher voltage battery to flow back round the loop to the other battery: subtracting itself from the incoming current to the higher voltage battery, and adding to the current to the lower. Thus current flows from one battery to the other. Both batteries will read the same voltage as they are connected by the still forward biased diodes, so it does not matter which one has the sense wire.

I suggest any sceptics do an experiment, you will see that what I have said is correct.

Diode, schmiode. Don't need to be lecturer to know that if you had one large bank on one side of the isolator and a small one on the other, that if the large bank was run down and the charge source was small, the smaller bank would NOT significantly discharge into the large bank enough to equalize the voltage. There is no magic energy to raise the voltage of the low bank just because there is a small charge source. The energy has to come from somewhere long enough (and/or fast enough) to raise the house bank voltage.

Lets say you have a 100ah start batt at 13.6V, and a 1000Ah house bank that has been run down to low voltage. Without a sizable charge source there won't be enough current to raise the house bank to 13.6V. It will remain at lower voltage until enough energy has into the house bank, or if the current is high enough (which could be quite a bit of current with a large bank).

Unless I'm not understanding some kind of magic that I've never seen in charging...

[bill good]

Deep Frz # 35 Only if the unit is wired correctly will it be an isolator!! Bensigler has 2 engines & only one isolator & here is where it becomes a bit of a can of worms. i.e. one starting battery or two? It has two alternators each charging what? So a bit more info along with the make & model of the isolators might be good. The ONLY way a diode can conduct is if the anode voltage is higher than the cathode (forward biased). So there is voltage on the input to the isolator when the engines are not working prob due to incorrect installation or wiring. Plainly not an isolator in this boat at this time. Note my boat has two of everything!!

[OceanPlanet]

Thanks to Bill for getting us back on track. Important questions.

[DeepFrz]

Yes, I was hoping to hear from the OP today to see how he made out.

[cal40john]

Originally Posted by nigelmercier

I'm sorry, but this is not true. I agree that this is not intuitive, so perhaps a little more detail will clarify:

Both diodes are forward biased, by the charging voltage. Being forward biased, their resistance falls, allowing some current from the higher voltage battery to flow back round the loop to the other battery: subtracting itself from the incoming current to the higher voltage battery, and adding to the current to the lower. Thus current flows from one battery to the other. Both batteries will read the same voltage as they are connected by the still forward biased diodes, so it does not matter which one has the sense wire.

I suggest any sceptics do an experiment, you will see that what I have said is correct.

You say both diodes are forward biased. You say there is a higher voltage battery, then you say they both read the same voltage? I hope what you are saying is that if the two batteries are at different discharge states, then their terminal voltages with nothing connected to them are different. When you connect them to a charging source, then both battery terminals are at the same voltage.

You say "some" current from the higher voltage battery (volts measured when unloaded since the voltages are the same when charging) flows around the loop to the lower voltage battery. Are you saying that the higher voltage battery is discharging? Are you saying that there are two current flows in opposite directions in the same wire?

Let's play another game. Connect a 14 volt source to one battery that is 1/2 discharged. The source is able to sustain the 14 volts so there is 14 volts at the battery terminal. Connect another 14 volt source to another battery that is only 1/8th discharged, again 14 volts at the battery terminal. Now short the two positive terminals of the batteries together.

Please don't tell me that you think that current flows from the 1/8th discharged battery into the 1/2 discharged battery. This is what I'm getting from you saying that current flows from the higher voltage battery to the lower. If the applied voltage is above the unloaded battery voltage, current flows into the battery. All that battery knows about the world is that there is 14 volts at its terminal. It doesn't know if it's all alone or if there are other branch circuits. Throw a couple of diodes in there and nothing changes. So even if your assertions about diodes were correct, there isn't any reason for current to flow backwards through them from one battery to the other.

John

[cal40john]

Originally Posted by daddle

Not so fast. That description is proper, as far as it goes. However I think charging performance may suffer if the regulator sense is connected to the well charged battery and the other battery is discharged. Most "smart" regulators I've met will sense the charged state of the battery and switch to low current float mode (moderate float sense voltage). Some do this by reducing the current for a moment. The discharged battery may charge very slowly, which may not be as intended. A simple charger may give better performance as it may just try to maintain some maximum sense voltage.

I don't believe I've come across a charger that reduces current as a test. Can you point me to an example? At first glance, I don't see how this would work.

Iota just uses time and voltage. Balmar uses duty cycle of the time it keeps the alternator on to determine how much the battery is charged. This only works well if you have programed in details about your battery bank. Guest doesn't do anything fancy, if you leave a couple of lights on it stays in bulk mode because it appears to be only going on current output.

John

[nigelmercier]

Originally Posted by cal40john

You say both diodes are forward biased. You say there is a higher voltage battery, then you say they both read the same voltage? I hope what you are saying is that if the two batteries are at different discharge states, then their terminal voltages with nothing connected to them are different. When you connect them to a charging source, then both battery terminals are at the same voltage.

Pretty much, yes. There are several simplifications in my explanation, due to internal resistance, wire and contact resistance, PN junction voltage drop... but the bottom line is that the charging voltage "turns on" the diodes, which then both become low resistance. As a result, current can flow between the batteries, this will tend to increase to incoming charge to one, and decrease it to the other. I wasn't trying to assert that there are two currents in opposite directions, just that there are two vectors involved.

By the way, I've been thinking about the sense wire, and I retract my statement that it doesn't matter which battery it connects to. During the majority of the charge cycle it doesn't matter, but during the end of the float phase (when the diodes become less biased, and begin to turn off) it does.

[Maine Sail]

Originally Posted by cal40john

I don't believe I've come across a charger that reduces current as a test. Can you point me to an example? At first glance, I don't see how this would work.

Iota just uses time and voltage. Balmar uses duty cycle of the time it keeps the alternator on to determine how much the battery is charged. This only works well if you have programed in details about your battery bank. Guest doesn't do anything fancy, if you leave a couple of lights on it stays in bulk mode because it appears to be only going on current output.

John

There have been very few regulators that even had a clue about current and those were mostly the old Link 2000R series. Balmar, Ample, Xantrex, Victron and every other charger I have worked on or installed in boats are voltage or time/voltage driven. They are clueless about amperage but they do know voltage.

If the battery is in bulk, which is any point below the absorption voltage, the alt or charger will be maxed out or bumping up against battery acceptance. With an alt in bulk the field current will essentially be maxed out and the only limit to current will be battery acceptance, RPM, alt temp or battery temp, on regulators/chargers equipped with batt temp sensors.

When the battery reaches the absorption voltage the regulator or charger then simply limits the voltage to that point. They don't bump amperage they only limit voltage. I always explain to my customers that alternator regulators should have been called voltage limiters and they then begin to understand.

If I had a dime for every customer who has done a DIY high performance alt upgrade then got pissed because their alt was only putting out a max of 45-50A into a 200Ah bank of wets when they paid for 125 amps...well...I'd be retired...

If someone knows of a marine charger or alt regulator that works on amperage vs. voltage I'd love to hear about it..