I have a fully set up ESS setup in place with a 300Ah battery behind a smartshunt.
I have another 200Ah I would like semi-permanently attached.
It would likely be left attached for long periods of time but I may remove it temporarily to use it elsewhere from time to time.
How would I best attach it?
I could attach the extra 200 behind my existing shunt and change it from 300 to 500 Ah but I’m not sure I want to play with calibrating that when attaching or removing the new 200.
I could just attach the new 200 to bus bar but I figure it would mess up monitoring and measurements so don’t want to do that.
Could I just get another shunt for the 200 and plug it into my cerbo? I feel it would give most accurate data and be simple.
I think my two questions are:
Is the second shunt a good idea? If so, what would you do when removing the 200? Unplug the shunt from cerbo? Just disconnect battery?
If adding a second shunt isn’t a great idea, what would you do?
You will need to make sure that the batteries have similar SOC when you connect the 2nd pair. If you have 2 shunts, only one shunt can be the monitored battery reporting SOC you would have to rely on both batteries sharing current in and out evenly. The ESS may well get messed up. One way around this would be to install the third party battery aggregator script. That is more messing than changing the Ah in the shunt.
Personally, having one shunt and changing the Ah is probably simplest.
I do exactly this with my 12V batteries. I’ve got a mix of camping batteries, trolling motor units, small 50Ah batteries for fridges, etc., roughly 10kWh of total capacity. I don’t use a shunt; instead, I rely on the Victron Multiplus to estimate the state of charge.
When I remove a battery from the system, it’s usually at the end of the day after a full charge. At that point, I upload a new VEConfig file with the adjusted amp-hour capacity to reflect the change. I run this setup with BatteryLife enabled, so the system ensures a near-full charge daily, which keeps everything balanced.
When adding a battery back in, I try to match the SOC with the rest of the bank. If the battery has a BMS with Bluetooth, I check its status or I’ll top it off using a small solar panel and charge controller. Once it’s fully charged and showing 100% SOC, I reconnect it to the system. Updating the VEConfig file remotely takes less than 30 seconds.
That said, I don’t always rush to update the config when adding a battery. Having too much capacity isn’t a problem; the only time it matters is when you remove one, then the system’s tracking would be off without the correction.
Smart shunt have only 1:1 serial interface. Although Cerbo offers more than one such interface nobody seems in Venus who is doing the aggregation of both values. There is no reason against doing this with only one smart shunt. The second (removable) battery is either a sink to discharge the first battery like a load or a source to charge the first battery like a mppt charger.
With other words: If a second battery is in parallel, the currents of the first battery go down equivalent to the capacities. SOC of both batteries is always similar and could be assumed equal if battery size and conditions are not too diffrent. On the other hand, I am sure Victron loves to sell you a second smart shunt. For using this as a spare part on stock its too expensive. My one never failed. Only make sure for the prerequisites what are same chemistry and same number of serial cells, similar size and condition.