Zamp Solar and BMK Issue ???

[Dutch Star Don]

I have a question WELL beyond my electrical knowledge.

A month or so after picking up the new coach, I added the Magnum BMK and ARC-50 panel to the coach. After some bad info from Magnum about where to plug the BMK into the charger, everything worked fine. The coach took about a day to get the SOC reading to change from "THINKN" to an actual percentage. When I did the BMK install, I moved all grounds to the shunt.

A couple months later I added three 150 watt Zamp solar panels and a 40 amp Zamp controller. I initially wired it with two large blade 40 amp fuses, one on the hot side of the solar wiring and one on the hot side of the battery wiring. Since they're so large, they're difficult to work with and remove the fuse. I replaced the fuse and installed a resettable pair of 40 amp circuit breakers they sell for automotive stereo. These ended up being cheap and were tripping from vibration. Finally, I added a pair of better quality 40 amp resettable Buss fuses that are working well. (Stuart W suggestion).

Now here is the issue. I installed the Zamp solar charger and ran the hot side directly to a battery post. The negative side, I combined with the other negative cables on the shunt. I pulled the plug on the BMK black box while doing the work. I reconnected the plug and the display panel went to "THINKN". I know it can take several hours for it to learn the system before changing to a percentage.

Well....after days, it didn't register. I called Magnum and they walked me through everything. When we got to DC amps out on the ARC-50 panel, while hooked to shore power, I was showing a fluctuation of -3.8 to -4.0 amps. Magnum said I had other issues and couldn't help. Everything was working and the batts were charging, so I gave up on it for awhile, which was last week when I installed the better quality circuit breakers on the solar.

As I thought about the issue, the only thing that changed was the adding of the solar. I decided to disconnect the negative battery terminal from the solar charger and connect it directly to batteries, thinking there was some back feeding going on. Problem solved!!!! Now the SOC was showing a percentage and that -3.8 volts went to -.8 volts which Magnum said was normal and what the inverter/charger would draw.

So now a question.....if the negative cable from the solar charger to the batteries is connected directly to the batteries, will I still get a correct reading on the SOC, since the negative cable of the solar charger is no longer connected to the shunt. The solar is only adding power, not taking any out, but does that mess with the math that the SOC is doing?

I asked a friend who knows nothing about RV's, BMK, solar, etc, but is an electrical engineer at JPL. His first thought was to add a diode to the negative solar cable allowing me to reconnect it to the shunt, if that is even necessary. (May not solve the issue).

Thanks

[redbaron73]

You will not be able to read the returned voltage without going thru the shunt. The result is you will show an artificially lower SOC than you really have, and down the road this could become a problem as the charger may try to overcharge.

First thing I would check is the shunt, make sure it is properly oriented. There is a proper direction for the 4 connections, and if any of the battery or sensor wires are wrong it will get confused.

If that is correct, a diode makes sense.

[Dutch Star Don]

Thanks... I'm pretty sure the shunt is correct as it works fine without the solar. I've installed two....pretty simple. I have a large bolt on diode, so I'll give that a try.

[redbaron73]

I am sure you know, but for others reading this:

By installing a diode on the shunt, then connecting the solar to the diode, the flow will only come from the solar, and will not return back.

There is a potential that the solar is direct back feed into the magnum inverter and will confuse the readings. The shunt has 2 more sensor wires that are to eliminate this confusion. This is no different than the engine alternator adding power to the battery. properly wired, the soc will account for the alternator added power.

[Dutch Star Don]

Red....okay, you concerned me with the diode issue. I had discussed this with Magnum before and got nowhere. I also didn't want to get in a finger pointing issue with Zamp and Magnum. This has aggravated me for awhile, especially since I don't have the skills to fix it.

After your above post, I decided to give Magnum a call and take a shot. There is one rep there that is not too sharp. This time I got one of the good ones. We tried a bunch of different things and he says a lot of my readings are backwards. He had me check my wiring, which I knew was correct. I know "Stuart W" is connected the same way I am and his shows different readings. The tech also said, that having the solar go directly to the battery would have the SOC reading wrong.....exactly what you said.

I told the tech I've had an inkling that the BMK module was bad. He agreed that could be the issue and is sending me a new one.

So until I get that, I guess this issue can go on hold. At first he told me that the module was $215 and I understood him to say that if mine wasn't bad they would charge my credit card. I told him to hold up as I could buy a new module on Amazon for $134. He explained, good or bad, they will only charge me if its been abused. That works.

[Dutch Star Don]

Well....after aggravating over this off and on for a couple of months, it's finally working. The black BMK module was defective. It was reading everything backwards when it came to amps used. If I ran the slide out, it would read +100 amps. When I was sitting idle, plugged in and fully charged, it would read -3.8 amps. I popped the new one in and everything is reading as it should.

Magnum shipped the new one in under five days, no charge and paid for return postage.

Thanks for the help and thanks to "Stuart W" who I bugged about this several times. I hope he doesn't add me to his blocked phone numbers.

[Stuart W]

Don this is great news! Glad it’s fixed, happy to help