Mastervolt ignoring Return Amps

[Trevor NZ]

I have a Mastervolt Chargemaster 75/3 which appears to ignore the Return Amps setting and will switch to float as soon as the min Absorb timer is reached. (I’m assuming the Return Amps setting % for Mastervolt is a percentage of the charger’s capacity, not the house bank, which it does not know).

There is a very small house load being a small DC fridge, but the issue remains even when I switch everything off. And I'd think if anything that could cause return amps to take longer to reach.

The other day it would repeatedly switch to float with around 18-20A still going into the batteries using the Mastervolt default of 15mins. Increasing the min absorb time to 200 mins has worked to fill the batteries, but I’d rather get return amps to work because if I say lose shore power then it resets the start times meaning I give the batteries another 200 mins of absorb when they don’t need it.

Any thoughts on what might be causing it? There are three batteries in parallel and there is a bit of cable run between them and the charger, giving us around 0.3-0.4v voltage drop. Not perfect I know, but I’ve accounted for this by bumping up bulk, absorb and float voltages to get the BTM and Balmar SG200 to read what I want at the battery when charging. Not sure if this voltage drop could be relevant?

Thanks in advance

[NewOrca]

Hi Trevor. The return amps setting is a percent of the charge current setting. Since your unit is dropping out of the absorb stage as soon as the minimum time is reached, it sounds like it is tripping on the return current. What is your charging current (for bulk) set to, and what is your return current setting? If the latter is too low (that is lower than your chronic house loads) then it will never trip—but that isn’t the problem you’re having. But if it is set too high, then it will be reached well before the batteries are charged unless overridden by the min absorb time. Do you have the Masteradjust app and a USB to Masterbus dongle?

[Trevor NZ]

Thanks for your response.

Have played around with Return Amps between 4.5-6% which, from a 75A charger, equates to between 0.5-1.0% of 642ah. So way less than the 18A or so it's actually showing at battery when it switches to float.

Bulk and absorb set to get 14.5v at the batteries. Battery spec's call for between 14.4-15.0. and most of our boating is over the weekend and then on shorepower for the week. Every now and again I charge them harder up to 14.8v.

Not sure it's relevant but I'm actually showing around a 0.5v drop between charger and battery at float, which is ever so slightly higher than I see in bulk and absorb. The other day the output at the charger was at 14.15v to get 13.65 at battery (which equated to 13.5 given the temp adjustment).

And yes, all done via the app and USB dongle.

[Stu Jackson]

Originally Posted by Trevor NZ

..................................

Not sure it's relevant but I'm actually showing around a 0.5v drop between charger and battery

...

That amount of drop, regardless of percentage of anything, should be considered unacceptable. Difference between full & empty is only 1.0V.


You should be alarmed (pi!).


Is the drop due to wire undersizing or bad connections?

[NewOrca]

That is quite a voltage drop, especially when the current is not that high. What's curious is that the drop is greater in float (0.5V) than in bulk or absorption! Do you have a diode splitter fitted? IFAIK the Mastervolt does not have a V-sense wire, so it only knows about voltage and current at its own terminals. Maybe you could get some insight by watching these values on the "monitoring" tab in MasterAdjust as the charger moves from bulk to absorp to float. Of course, there could just be a fault in the charger :-(

[Trevor NZ]

Thanks, yes the drop at float seemed odd and I'm sure greater than in absorb and bulk. Nothing can bypass shunts so the two gauges (which are aligned) should be right.

The Chargemaster doesn't have sense wires (think that might be the next model up).

I'll try and do a full audit this weekend. There is a fuse by bat charger but otherwise nothing else. The only thing I can think of is that there is a switch to parallel the house and start banks, which is set to off, but could that be an issue? **EDIT: This is tee’d into one battery as a short run to the switch, which connects to start bank which is powered off the port alt, if that makes sense. That house bat is connected to the next house bat directly, not via this switch.**

I should add that the drop I'm reporting is based on the monitoring tab of the MasterAdjust software not the charger terminals. I'll try and do the whole lot this weekend and compare:
reported voltage at charger in software
actual voltage at charger
then go to fuse
then each battery and back

It's a long story as to why, but there would be I guess 8m of cable as a result of charger and bat locations. I asked installer to overspec when we re-wired to make it a loop with -ve and +ve at opposite ends, given length of run, but that's another thing for me to check, using the MV guidance on length and cable diameter.

Thanks

.....

[Trevor NZ]

The plot thickens. I’ve just had an engineer friend who was onboard check and the bat charger terminals matched the voltage readings from the shunts. So the good news is there is no voltage loss…we also checked and the same was true with the charger off, and also in bulk.

However, this means the batteries had comes up to 14.15v in float, which matches the charger setting (because I increased it yesterday to get the batteries to my target float voltage of 13.6 in the colder temp). Not ideal, but they would have only been floating at that for 18 hours or so.

He didn’t have the laptop to check what the charger thought it was outputting, but the voltages he recorded were what I have set the charger to (with temp compensation).

SO – that leaves me with the issue of the Charger still ignoring Return Amps, AND a weird situation yesterday when batteries in float were reading 13.6v and charger was outputting 14.15 (with between 0A-1A going into them so they were completely full)...

[goboatingnow]

8m or 16m round trip is a lot of cable without voltage sensing

I’d suggest the return amps timer is tripping out first.

You need to measure the cable drop and confirm that is not an issue.

[Trevor NZ]

Hi

Thanks for this. We have now measured and voltage drop does not appear to be an issue. See details above.

It's also an approx 8m round trip. I don't know exact size of cables, but they are substantial.

Thanks
Torrin

[NewOrca]

The plot does thicken indeed. It's puzzling that you and your engineer friend now find no voltage drop at all even in bulk (what was the current?) with 8m of cable in series. It might be worthwhile to make an accurate diagram of your system showing the locations of the charger, shunts, batteries, switches, fuses, and any other loads or connections. As the old joke goes, there has to be a pony in here somewhere :-)

[Trevor NZ]

Thanks. Just to be clear, the batteries are wired in parallel.

Not sure of current but batteries were completely full so that may have had a bearing on it. I think historically I’ve seen maybe 0.1-0.2v drop at absorb on a normal charge. But I’ve not compared charger terminals to determine that but rather those figures are based on the assumption charger outputting exactly what I’ve set it to. Given those readings are generally when we’re out cruising or just back at marina when batteries are warmer it could be partly temp compensation.

[smac999]

All chargers are timmer based. They have no way to know net battery amps. they all go to float to early if batteries are low. Just flip the breaker and reset it.., Same with external alt regs. You can just stop and restart the engine. And bulk timer will restart.

The magnum inverter charger with the magnum battery monitor and advanced screen may be the only exception.

[goboatingnow]

Originally Posted by smac999

All chargers are timmer based. They have no way to know net battery amps. they all go to float to early if batteries are low. Just flip the breaker and reset it.., Same with external alt regs. You can just stop and restart the engine. And bulk timer will restart.

The magnum inverter charger with the magnum battery monitor and advanced screen may be the only exception.

Not so, the victron smart range of chargers can measure tail currents via an attached Bluetooth BMV 712 or victron SmartShunt

[Trevor NZ]

We have run further tests: the charger outputs, batteries, shunts and displays are all consistent. There are two seemingly odd things though:
1. The battery charger terminals are reading around 0.4v below what they should according to the settings I have put in. While this could be an error in the tester, the tester accords with two separate shunts and so I’m inclined to think it’s okay.
2. To get the batteries to float at 13.5v I have set bat charger to 13.9v. All is fine but when I go to boat (batteries are completely full and at 13.5v on display) and start a small load, e.g. fw pump, the voltage jumps to around 13.9/14.0v and then drops to 13.5 when the load is removed.

Any thoughts?

[NewOrca]

Maybe there's a clue here. Can you post a screenshot of your MasterAdjust Configuration page? Or list the values set in the second column: Bulk--Voltage, Min time, Max time, Start bulk time (V); Absorption--Voltage, Min time, Max time, Return Amps; Float--Voltage, Return to bulk (s), Return to bulk (V)? Also the settings for Inverter Energy save mode.

At some point you might want to contact MasterVolt tech support--I've found David Freeman there to be very helpful. Here are their coordinates:
Technical Service
800.307.6702 direct
N85 W12545 Westbrook Crossing
Menomonee Falls, WI 53051
tech.mastervolt@oneasg.com