#62490  by bramwell
 Tue Oct 15, 2013 4:12 pm
When you say a BMS would have definitely prevented this could you elaborate please. Is that with a user hovering over the BMS and intervening in some way or does the BMS manage the whole process automatically? If so how?

Thanks, Bruce
 #62494  by Nut17
 Tue Oct 15, 2013 4:31 pm
Hi Bruce, this small device is all that is required by AA Solar http://www.aasolar.co.nz/Votronic%20Bat ... puter.html to comply with their 10 year warranty requirement. A relay which is controlled by the Votronic computer can be pre set to cut the load at a pre determined state of discharge to prevent the potential damage caused by completely draining the battery.
 #62503  by newPowr
 Tue Oct 15, 2013 7:02 pm
Hi Bruce

A BMS has 4 key functions: ensure the battery doesn't get over discharged, ensure it doesn't get over charged, balance the SOC of the individual cells and, partly as a corolary to the previous, ensure the individual cells in the pack don't get over charged or discharged either. In most systems all functions are fully automatic and requi no user intervention, though simpler systems may rely on audible alarms or the like for some of it.

How specifically each function is achieved depends on the type of BMS used, which in turn depends on the system requirements and application. Comparatively simple systems like these use either an integrated or distributed BMS with top balancing (whereby a shunt comes in action once a cell is charged allowing the others to catch up). Very deep cycle batteries with heavy use sometimes use bottom balancing which can be faster and more efficient. Traction batteries and larger systems generaly use advanced systems with CAN bus control of charging and discharging, mission critical systems use BMS's with multiple layers of redundancy and staggered levels of availability/service, etc. etc. Whatever the system the common denominator is the BMS measures the voltage of each cell (and on the more advanced temperature and SOC) and drives the system on that information.

Chris

It should be evident from the above that a simple low voltage cut off can't possibly achieve all this. At best it does part of the first function and only at an aggregate level, and I'd personally be worried [if anyone] thought that's not needed either (in a "fooled me once shame on you, fooled me twice shame on me" kind of way)
Last edited by Mark on Wed Oct 16, 2013 9:14 am, edited 1 time in total. Reason: Edited by Mark
 #62507  by Justintime
 Tue Oct 15, 2013 7:43 pm
Hi Florin, how do I know if one of the BMS circuit boards has failed or is not operating correctly? Because that would put me back to square one, wouldn't it. One thing that came to mind from your last post is that mission critical systems have redundant systems - as far as I can tell, I don't!

Cheers, Dene
 #62529  by newPowr
 Wed Oct 16, 2013 9:05 am
Hi Dene

I think calling a RV house bank mission critical is pushing the argument too far to the other extreme (considering the discussion started with whether a BMS is needed in the first pace and the replaced lead had no such requirements either, presumably). A mission critical system by way of example is the electrical system on the Dreamliner (since controls are now fully electric instead of hydraulic and without power the thing falls down from the sky). The solution they chose if curious is very elegant and robust (most people don't know but the battery is not actually providing any current in flight - that's given by the generators on the main and aux engines, and the plane can fly with just one of the three running. If all three fail - one's bound to ask - the plane can deploy a folding "windmill" generator and that can carry the critical system loads on it's own). Anyway, more down to earth mission critical system could be the navigation lighting/equipment on a boat or UPS for datacentres or life support.

In principle, the BMS boards are designed to fail "safe" (meaning a failure is interpreted as a cell failure and therefore very conservative). the "one in a million" exception to this (which is a possibility you would only consider in cases as above) can be subsequently minimised by multiplexing (e.g. we can add two independent measurements of each cell - this would drop failure of the BMS subsystem to one in a million millions) or other methods. For your bigger system for instance we can opt for parallel redundancy for extra survivability, or similar. The good news is that reliability engineering is an exact applied science and if you can define a target there are precise methods and controls to ensure the desired outcome.
 #62531  by Roamnz
 Wed Oct 16, 2013 10:25 am
From a purely lay persons perspective and from some one who has been living off the grid for a good few years now, with lots of appliances and gadgets on board for everyday living, after reading all this technical stuff in this thread I think we will continue to stay with our KIS system. Is it just me or do a few people who are not electrically trained, find all this a load of mumbo jumbo? :-| Life is too short to bother with stuff that is so complicated! We have proved as many others have proved, living off grid in a well set up motorhome, with adequate power can be so simple, one has to ask why do the 'experts' out there make it sound like rocket science!

The 4 x 120 solar panels, 1 x 45 Morningstar MPPT solar controller, a votronic monitor, a Xantrex 1800i inverter and 2 x 260 amphr AGM batteries, this combination works so well for us. Living full time on the road in a motorhome, I need to be able to turn the inverter on and all my power points revert to 240 allowing me to plug in the toaster, the coffee machine, the hair dryer, the hair straightening tongs, the kitchen whizz, the slow cooker, the vacuum cleaner, a steam iron, and that is just for starters. Of course I know I cannot turn all the gadgets on at the same time but that is a small hardship for the privilege of being able to live this lifestyle.

We recently replaced our batteries because they were starting to show their age with the same as what we had for a few dollars over $1000.00. We read long and hard about Lithium but it was exactly what has been demonstrated in this thread that turned us against them plus the ridiculous cost here in NZ. We have been parked stationery at Ardmore for 6 days, alot of awful weather and grey skies up til today,
( we have both had this nasty cold bug that is doing the rounds) at 9.25am this morning our Votronic monitor showed the batteries back up to 520, the morningstar solar controller is green blinking and the other monitor shows that there is still a small amount if solar pulsing or trickling in, so from my basic understanding of a good power setup it is all doing exactly what it is meant to do!

Possibly when Lithium batteries and all the add-ons come down in price to a competitive level and when the suppliers realise that blinding the average person with science may be losing them sales rather than gaining them new customers, we may look at Lithium, but til that time I am glad we made the decision to stay with what we know and trust through our own personal experience. :)
 #62532  by newPowr
 Wed Oct 16, 2013 10:37 am
Don't think that's a fair comment - this isn't any more complicated than a smartphone or some other device you might be using.
Some people are curious to know how things work (hence asking and being answered).
Some don't and for them suffice to say the system can be fully automated so it's completely transparent in operation.
 #62538  by WoodyZ
 Wed Oct 16, 2013 12:07 pm
If I have a battery systems failure on the road and freedom camping, the beer and wine would get warm.

Mission Critical? Hell yes.
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