Fluctuating Battery Voltage

[itsallgood]

I have a TriStar MPPT 60 charger attached to 2 lithium batteries. When there is no load on the batteries and controller charging, the voltage jumps all over the place, 13v to 20+v. Placing any load on the batteries settles everything down and everything works as you would expect. Morningstar sent me a new controller with no difference. MS tech support is at a loss after trying many things to rectify. The battery supplier isn't answering my calls for help.

This system is all new and behaves this way from day one. Is this behavior an indication that I have not wired things correctly? The solar system is isolated from the on-board systems using manual isolation switches (only one dc source can be active), except for the grounding which is common with the on-board dc ground bus.

If anyone has any ideas I sure would be thankful.
Steve

[Bama Rambler]

Does Morningstar say that the controller is compatible with your particular Lithium batteries?

Have you tried temporarily swapping the batteries with batteries of different technology to see if the voltage settles down?

How are you reading the voltage (multimeter, display, analog meter, etc.)?

[itsallgood]

Originally Posted by Bama Rambler

Does Morningstar say that the controller is compatible with your particular Lithium batteries?

Have you tried temporarily swapping the batteries with batteries of different technology to see if the voltage settles down?

How are you reading the voltage (multimeter, display, analog meter, etc.)?

Well they haven't specifically said they do or don't support mine but on their website they do claim to be compatible with lithium technologies in general but there are so many technologies, more so on the BMS side that at this time they don't claim to support specific brands. The support fellow I was working with tried my returned unit on flooded batteries with no issues but I haven't tried it myself. I need to ask him if they might try my brand of battery. I have a TM2025 meter which shows the voltage fluctuating and I have also used a digital meter across the batteries.

[Bama Rambler]

It may be the internal resistance of your particular batteries, especially since adding a little load levels it out.

[Bocamo]

Lithium ion batteries are very finicky regarding recharge. Overcharging can rapidly deplete the ion-matrix and most chargers for lead acid batteries shoot for about 14 volts or so. I see nothing in the Tristar manual referring to lithium, rather there are specific settings for Gel, sealed, flooded and AGM. Elite Power Solutions deals with lithium batteries for this market and they might be able to offer some advice.

[Flybob]

Do the lithium batteries have internal charge/discharge/balancing circuitry. If so, this can cause conflicts with the morningstar controller as they fight each other in a sense.

[itsallgood]

Originally Posted by Bocamo

Lithium ion batteries are very finicky regarding recharge. Overcharging can rapidly deplete the ion-matrix and most chargers for lead acid batteries shoot for about 14 volts or so. I see nothing in the Tristar manual referring to lithium, rather there are specific settings for Gel, sealed, flooded and AGM. Elite Power Solutions deals with lithium batteries for this market and they might be able to offer some advice.

No, lithium isn't mentioned in the manual but they cover it some on their website and the BMS could be the issue. Using the custom programming feature of the TriStar I set up the charging profile as suggested by the battery manufacturer. The battery manufacturer says they are familiar with these chargers and said there are no known issues (taken w/a grain of salt). They recommend charging at 14.6 volts and never to exceed 15v. The TriStar MPPT can be easily programmed to meet their charging requirements but obviously it is being pulled to higher voltages. It's almost like the TriStar has no reference voltage.

[itsallgood]

Originally Posted by Flybob

Do the lithium batteries have internal charge/discharge/balancing circuitry. If so, this can cause conflicts with the morningstar controller as they fight each other in a sense.

Yes these batteries have built in BMS. The support tech never touched on it but their website does mention that some BMS's can cause erratic behavior. Honestly, I would have thought MS would be dealing with these issues as this is not entirely new technology.

[itsallgood]

Originally Posted by Bama Rambler

It may be the internal resistance of your particular batteries, especially since adding a little load levels it out.

If that is the case, is there anything I can do to counteract this? It seems that to keep a constant load on the batteries would solve this but I'm not yet sure I want to have to do this and I don't yet know how much of a load is required to stabilize the voltage.

[ScottBrownstein]

What do you mean by "only one DC source can be active?" You mention the TM2025 so presumably it is connected to the neg and pos posts of the batteries. Why do you think your need isolation switches?

[itsallgood]

Originally Posted by ScottBrownstein

What do you mean by "only one DC source can be active?" You mention the TM2025 so presumably it is connected to the neg and pos posts of the batteries. Why do you think your need isolation switches?

The main reason is so that when plugged into shore power, there aren't two charging sources active. The second reason was to prevent any feedback into the onboard charger while on solar. I may have misunderstood all that I read but this is the recommended way. The MPPT is used for the solar charging side so when on solar it is switched to power the dc panel directly isolating the on board charger.