Renewable Energy Questions/Discussion > Automation, Controls, Inverters, MPPT, etc

Battery voltage sensor placement TrisStar 45 MPPT

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So yesterday I put in my new TS 45 MPPT. All wired correctly. Powered it up and t appeared to be working just fine(there was no sun at the time)
System details (12V battery bank 6 GC2 batteries, 200W solar on this controller (going up to more shortly))

The TS  has a battery voltage sense option,  that allows you to connect a small pair of wires to the battery bank to improve accuracy over the built in voltage sensor. I had added this and 18g wires that went to 2 battery terminals.

At power up, the TS voltage, matched my meters, and my monitoring system, all is good.

Later on in the day, when the panels were in full sun, I look at my monitoring system, and the voltage is at 15.5, so I go look at the TS thinking its trying to equalize or I have set the wrong parameters for the battery. It reads 14.8 V. Huh? , a .7 Volt difference from the meters and monitoring system. I know the meters are correct. The TS must be reading wrong.

Then I work it out, I had placed my voltage sense contacts on the same posts as solar input lines from the TS where current was flowing, that was creating a voltage drop on the voltage sense lines.

Today I moved the voltage sense contacts to unused posts on the battery bank. Now everything is reading the same . Moral... watch where you put the voltage sense contacts. In this case if I hadn't rectified the problem I would have been continually charging at a higher voltage than spec, probably not a good thing.


Glad it works..... but I don't quite understand.

When you mentioned the .7v I figured a diode in series somewhere.......  but not that.... so:

I would have thought that putting the sense lines on  the input posts would have given you the opposite effect..... ie it would be at the highest EMF of any part of the bank when charging.... wondering why you found it to be the lowest by .7v....

Does not compute in my tiny mind.

I would have thought that the input posts would give you an artificially high reading in charge mode.... not the opposite.


It didn't make sense to me either, and I never cosidered the possibility of something like that when placing my voltage sense wires. So of course I dug through the TrisStar manual (which is a good manual). 

And although the section I read, related to why you should use the separate voltage sense as opposed to the onboard, was that any current carrying wire will cause a voltage drop. So if you used the onboard (sensing from the controller output) there would be a voltage drop.

Maybe, what I did was not related at all, maybe it was just moving the sense wires, maybe better contact , etc. But I find that unsual if so as tehy were clamped down the same way on the binding posts.

From the manual
"Voltage drops are unavoidable in power cables that carry current, including the TriStar MPPT
battery cables. If Battery Sense wires are not used, the controller must use the voltage reading
at the battery power terminals for regulation. Due to voltage drops in the battery cables, the battery
power connection voltage will be higher than the actual battery bank voltage while charging
the battery.
Two sense wires, sized from 1.0 to 0.25 mm2 (16 to 24 AWG), can be used for battery voltage
sense. Because these wires carry no current, the voltage at the TriStar will be identical to the
battery voltage. A 2-position terminal is used for the battery sense connection"

It is is quite possible that I have misinterpreted the whole thing.

0.7V is a lot.  Guessing a bad connection somewhere.

Was there a load on the system?
Charging current goes one way.
Load current goes the other way.
Combined, the result could be either way.

The sense wire battery terminals are soldered, and the sense wire itself is soldered solid where it screws into the TS45, right?

It doesn't take much to throw if off a tad.
In the ghurd controller, the Yellow LED (dump load is dumping indicator) current (3 ~ 3.3ma) causes enough voltage change in the sensor wires that it had to be included in the design calculatons.

Hi Ghurd,

At the time there was no load on the system. Charge current was probably around 10A.

The sense wires are nothing more than 18g wires with crimped on ring terminals to the battery posts, on ts 45 the wires go into screw terminals.

Funny thing is that now that meters all jive, really not worried about, it could be a bad connection or what not, but it was funny that only when the TS 45 was charging was there a difference.

Now I'm just worknig out how to get the TS 45 to play nice with the Blue Sky... (actually it may be playing nice just seeing behavior I'm not used to)


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