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Terminating Contact Resistance

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Dotnet

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Hello All,

A quick bit of background to my question -

I'm just finalising an installation of a split charge system on a narrow boat comprising of a couple of AGM battery banks (start & auxiliary).
I have one 'Voltage Sense' wire from a charging regulator which is designed to go directly to the +ve of the battery being charged, and will monitor to 2 decimal places (100ths)

Now my question:-
I would like to monitor both batteries by inserting a very low contact resistance DPST relay and splitting the connection. So, what would be the ideal way to terminate the connection so that I loose the minimum due to volt drop?

E.g. with the wire from the manufacture I already have two 'connections', one for the wire to the crimp (not soldered) and one for the crimp to the spade connector. So if I go the crimp route I will have another two as above to the relay, 1 through the relay contacts, another two coming out and finally another two to the battery terminal. - A total of nine locations of possible volt drop (per battery not doubled up because there are two batteries)

Or

Nastily, I could (silver) solder directly to the pins of a PCB relay and have just 7 if

I really welcome you thoughts/input

Thanks
Matt
 
There should not be any significant current flowing though the sense wire so a small amount of resistance in that wire should not be a problem. Just having a positive sense wire does not take into account the voltage drop in the negative lead between the alternator and the battery. A better design would have two sense wires, one positive and one negative. When you say that you would like to monitor both batteries do you mean at the same time or by selecting which one to monitor with a switch ?

Les.
 
Hi Les,

Thanks for reply.
The kit I'm using is a Balmar Alternator, Balmar ARS-5 regulator and a proprietary voltage sensing relay system (which is causing me the grief)

Noticing the ARS-5 had no dedicated sense wire I did some hunting around and found the following post which, very eloquently, explains what's on my mind and why - have a look at 'Maine Sail' (near the top) in **broken link removed**

So for the sake of £30 I will be upgrading the regulator to the unit mentioned in the article BUT it only has one sensing input - hence why I'm considering switching the sense wire over when the banks get switched. I have quite a few connections plus a length of run issue not to worry about boiling a battery.
The other option could be an Op-Amp to deal with the losses but my analogue electronics are not up to that level.

To answer your question specifically, I want to make sure each battery is protected / monitored during it's own charge. When charged the two bank will operate as one

Matt
 
I assume that you are going to make sure that the high current switch to select which battery is connected to the alternator is interlocked with the switching of the sense wire. If the sense wire is connected to the battery not being charged then it will overcharge the battery.

Les.
 
I would mount the high-current battery selector (Off, 1, 2, or Both) near the two batteries so that the high current wiring can be short, and then wire the sense wire to the common (output) terminal on the battery selector. That way it is idiot proof. The voltage drops in the selector switch are negligible (except when cranking the V8 in my boat).
 
I agree with MikeMl's solution. One other point is you say in normal operation the batteries ate in parallel. I would have thought it better to have one for engine starting and another for all other loads. With the batteries in parallel if the pair become discharged then how will you start the engine to recharge them ?

Les.
 
I agree with MikeMl's solution. One other point is you say in normal operation the batteries ate in parallel. I would have thought it better to have one for engine starting and another for all other loads. With the batteries in parallel if the pair become discharged then how will you start the engine to recharge them ?

Les.

In my Cabin Cruiser, both batteries are flooded-cell, lead-acid; one is a deep-discharge house battery (for running cabin lights, water pump, the stereo), the other is a starting battery. In a pinch, the house battery will crank the V8, but the starting battery has higher CCA.

Normal operation: both off when on the trailer. 2 for starting. 1 for camping. Both when cruising. Since both batteries are similar in chemistry, the alternator voltage regulator holds the selected battery (either or both) at ~14.4V during recharging.

My boat sits on its trailer (under roof) for months at a time with no AC line voltage available. I use a solar panel charger to "float" the batteries independently.

To enable periodic "equalization" of each battery, I use the alternator during cruising. I tricked the alternator controller to temporarily raise the charging voltage (using a toggle switch) to ~15.2V, which is sufficient to cause the cells in the selected battery to outgas as recommended for equalization of the cells.

The sense-wire for the alternator controller samples the voltage at the common point between the battery selector switch as I indicated. The voltage selection switch (normal charging or equalization) was put inside the feed-back control loop inside the voltage regulator. Now there are two trimpots that set the output voltage.
 
I agree with MikeMl's solution. One other point is you say in normal operation the batteries ate in parallel. I would have thought it better to have one for engine starting and another for all other loads. With the batteries in parallel if the pair become discharged then how will you start the engine to recharge them ?

Les.

Apologies - the engine start battery has priority then the house bank switches in therefore in parallel. But here lies my problem when the VSR's switch over the voltage sense will then not monitor the 'house' batteries correctly. It will 'see' the (parallel) voltage on the start battery and the volt drop across connections termination of the system. I want the monitoring to be connected directly to the +ve post of the battery (bank) being charged so it takes into account the losses.

I very much appreciate the input whilst the points that you mention are all valid they have been addressed - but could we go back to my question - contact resistance.

The only point I have to deal with is the single cable for battery monitoring which needs to be split to two battery banks, and I'm trying to find a way of incorporating a very low contact resistance SPDT relay (or similar) into the sense wire so that it impacts minimally with the operation of the motoring.

Thanks
 
I predict that the voltage drops in the selector switch and wiring are insignificant during normal battery usage. Have you actually measured them, or do you just think you have a problem?

Why not just diddle the VR to compensate for the couple of mV you are likely to measure?
 
I agree with MikeMI's comments. After finding out what VSR stand for in this context my understanding is that the VSR is a high current relay with change over contacts with some voltage sensing electronics to decide when it switches the cable from the alternator from the start battery to the house battery. If you can access the connections to the coil in the VSR you could connect the coil of a small SPDT relay (5 amp contact rating would be plenty.) in parallel with it. this way the small relay would change over the sense wire between battery terminals. You could also just use a second VSR to switch the sense wire between batteries. (This is very much overkill using a 140 amp relay to switch less than an amp on the sense wire. You could also design you own VSR with just a low current relay. If you choose this option I would suggest trying to get hold of a faulty VSR and reverse engineer it rather than designing one from scratch.

Edit The idea of using a second VSR would only work if the voltage sensing is done on the terminals that connect to the batteries.

Les.
 
Last edited:
Chaps,

With respect you are telling me things which I already know AND advising me on things that are already being done as all the kit is off the shelf (pretty much the same as a design I came up with)

My issues is when the control circuitry switches over from the start battery to the house battery bank and you have a fully charged battery in parallel with a semi discharged bank whose voltage will average out and give an incorrect reading to the charge regulator. As we are talking about 100mV between the various stages of charging with AGM's I want to split the charging and monitoring to the individual banks until they have both reason the 'float' stage where by they will then be paralleled again as per design.

So again could I advised on the best method to split the voltage sense wire that has the minimum loses bearing in mind the wire has to be physically terminated hence the question crimp or solder or solder and pcb

Matt
 
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