I don't know what you mean when you talk about mixing BB and Renogy - batteries or equipment. If you mean batteries, then yes, you can not mix them. But if you mean BB batteries and Rengoy DC-DC Charger, then you should be able to mix them as I did so before I switched to a Victron DC-DC Charger.
That is certainly not what folks like BB are telling their customers. I was told that I could not use any other Lithium batteries with the 2 BB that I have, and I assume that is because the BMS of other manufacturer's batteries are different from the BMS that BB uses. And I assume that applies to all Lithium batteries that have their own BMS.
BB also tells folks to charge at 14.6V. They are not an authoritative source in my opinion.
Any battery manufacturer has an ulterior motive to avoid what is known as a pi**ing contest. If you have batteries from different manufacturers and there is a problem, they each point fingers at each other and at you and it can get heated. It is a support nightmare. Technically, it is possible that they perform differently under extreme overvoltage and undervoltage conditions such that a corner case results in a lockup or similar problem that does not resolve itself without intervention. If you really understood what each battery's BMS does, you should be able to have them work together, but most people are not battery experts and they don't always publish full information.
Agreed, more than 4 parallel strings should be avoided. But it really depends on the loads.
I would think that they would be pretty authoritative concerning their own batteries. And, in any case, they recommend that my Victron DC-DC charger be set to charge at 14.2 - 14.6 volts. Here is a cut-and-past from the FAQ on their webpage:
I am pretty sure there is some of that, but they also tell you that you can add new BB batteries to old BB installations for up to 2 years, so they are not telling people to buy completely new sets if someone wants to add another battery.
I have a Victron 30 amp DC-DC charger, but that current gets added to any solar input so when driving it is not unusual for me to see 40 or 50 amp charging. 50 amp charging with a 200AH system is only 25% at max so that is right at your lower limit. On the other hand if I added one more battery it would only be about 17%. Personally I would like to be sure that you are right and that the built-in BMS would not be an issue.
Out of curiosity, why?
They did not say. Maybe @Midwestdrifter has more insight. I do notice that I have to get them to balance once in a while and they don't all balance at the same time. Some are finished first. Maybe it is more of a guideline than a hard and fast rule.
My batteries each have a Bluetooth interface, so I can look at individual charging currents and cell voltages. So far, the cells are pretty well balanced in a pack. Charging and discharging currents vary the most under low current conditions, some batteries may stay in standby while others share the load. Under high currents (300 A discharge) they still vary +/- maybe 20%, maybe between 60 and 90 Amps per battery. The wiring resistance is not perfectly balanced due to mounting locations requiring longer wires for some.
Since my wife and I do a lot of boondocking and because the Regency Ultra Brougham we now have seems to use a lot of power (just running the inverter eats up about 2 amps/hour even if you are not using the outlets, and none of the usb charging connections work without the inverter running, the macerator toilet uses a fair amount of power, and so on) I have thought seriously about adding another battery. The problem is that the battery box only has room for 2 type 31 batteries and so I would have to add a 3rd battery somewhere else and thus have to have the whole connection wiring redone to balance the distances. In addition my current BB batteries do not have any bluetooth connectivity so I would have no idea what was going on inside the batteries. Given all of that you comments are very interesting.
Exactly. I regret that I did not know enough about this when I first made the switch to Lithium. There are a lot of threads about upgrading from the OEM batteries to Lithium and they discuss a lot of issues, but I never saw this being discussed. The Victron SmartShut is much better than nothing, and I always have some idea of how much power I have left, but I have no idea what is going on inside the batteries - if they are balanced, if any of the cells might be going bad or even if one of the two batteries has failed.
You can compensate somewhat for wire length by using larger wires. 4/0 is a practical size limit and I have some of that, along with 1/0 to most things.
That can't be assumed. The original installation in mine had a 3000W inverter and woefully undersized wiring to the batteries. Use one of the online wiring calculators and figure it out correctly. I would use a 1% voltage drop for battery to inverter rather than the 3% default on many calculators. It will keep things from crapping out when the batteries are low.
I have thought about doing exactly that but keep coming back to the fact that doing so would leave me with only a single battery and if something happened to that I would be left with no battery power. No second battery to use in case of the failure of one.