After 100’s of lithium systems with a variety of BMS’s, we determined the shortfalls in nearly all of them when there is an extreme scenario. We then designed our own BMS and used it first up in Meteor battery range. It has been a huge success. Here are the design points that make it quite different:
Current sense is less than 0.1A at 48V.
When you put a large bank of batteries together, low loads are shared between the batteries. If the current sense resolution in each battery is say 0.5A, then 10 batteries in an array wont measure outgoing power till 5A at 48V is reached. The quiescent DC power for most Victron Systems is around 12W with modem and screen connected. The standard BMS will fail to read this. The Safiery BMS will read it down to 0.1A per battery.
BMS Cut Off voltage is just below 0% SOC
On the surface, this seems quite sensible. If the BMS doesnt turn off, it will continue to drain the battery. However, if it does turn off, then recovering a flat battery safely with BMS fully functional is so important. Otherwise batteries have to be disassembled and cells charged independently.
The Safiery BMS turns off around 8VDC. Ina typical 48V battery there is around 2-3 weeks before this will fully drain the battery. However, when the customer becomes aware of the flat battery bank, Scotty AI or an AC charger can recover the bank safely to 100% health. This small change eleiminates the majority of calls for “help” from owners without it.
Stop charge a Marine Regulatory requirement that has a problem with Victron Compliant BMS unless they follow this exact guideline from Victron
Victron compliant BMS guidlines call for just 3 parameters to share on the CAN BMS. Charge Voltage Limit, Charge Current Limit and Discharge Current Limit.
Most (nearly all) the BMS’s we have used in the past send a zero value for the charge current limit to “Stop Charge”. The problem is Victron does not always respond promptly to this command. The Victron External BMS commands respond immediately to voltage though. We had a horrific experience with this when a high power solar bank continued to charge the 48V batteries for 15 minutes after the BMS had issued a zero Charge Current Limit. That was on a catamaran. Lesson learned.
Safiery’s BMS sends a zero Charge Current Limit to Victron some 3-5 mins before the battery relay may open. Then 1 minute before the battery relay may open it reduces the Charge Voltage Limit and Victron responds perfectly. The timing to reopen is internal information. It has taken more that 5 firmware revisions when testing to perfect this.
In the next accordion block is an on-line review of a customer who went away (to Bali) for 2-3 weeks with the system on and no solar charging as the vehicle was moved to the garage.
Watch to see what happens. This is truly the best example to give confidence to customers to manage their sysems by them self, an absolute essential in marine.