Posted by Peter Kennedy on 11/14/2016 to Battery Monitoring
I'm getting weird reading from my Victron BMV amp hour meter. The readings don't seem to relate to what I think is happening. HELP !
I would first like to make sure that your meter is measuring everything coming in and out of the battery. If you have anything connected to the battery negative other than the shunt then it wont get measured. This is the most common installation error so if there is anything wrong with this then none of the readings will make sense.
Next up we need to see if since you installed the meter the batteries ever got to full. What happens when you first connect it is that it assumes the batteries are full. Then if in fact they weren't full and get charged up a bit more then it resets and assumes the new point is full. This can continue for some time until they actually do get to full and when that happens reality and what the meter says coincide. In the meantime your battery charger, alternator, solar charge controller or other charge sources are using their own rules to decide how to charge the battery. Since they are not talking to each other they may come up with different answers. The best way to deal with a new installation is to put it through its paces in such a way as you can verify what is happening. So in your case it would be good if you could use a battery charger to get the batteries to full and then verify that the meter recognized them as full and then pay close attention to what is happening after that.
You also have to understand how this all works so you can understand the limitations of the system. The amount of energy in the battery is variable and depends on the temperature and on the rate at which you use the power. To deal with this the meter has to use some fuzzy logic. The principal way it does this is that it measures cycles. Once it decides the battery is full and then it discharges at least 20% and then back to full again is how it defines a cycle. At that point it resets itself to say the battery is full. If you never actually do a cycle then the errors caused by the variable performance of the battery with temperature and rate of use accumulate. It is only when the meter resets that the errors are erased and you are good to go again. You can look in the history menu to see how many cycles you have done. It uses the performance of a cycle to measure the battery efficiency by comparing how much power was taken out versus how much was put back in. It assumes the battery efficiency is 90% to start but learns what it actually is over time. It is only allowed to change the efficiency value by 1% for each cycle So If you would like me to follow up with this again I would ask you do the following: 1. Show me a photo of the installation showing the shunt and where it is connected to the batteries so I can see that you have no wires bypassing the shunt 2. Sent me a copy of the historical data which you will find in the historical menu, as well as any setup data you entered 3. Put the system through a full cycle and record your readings including voltage, amps and amp hours measured