I've never been very keen on the idea of working around a bank of Secondary Cells. I usually wear as much PPE as I can possibly get on when working near them. It's not the large fault currents that worry me should a short-circuit occur from dropped tools and the like, it's the threat of the cells actually exploding during a short-circuit and releasing thier Sulphuric Acid. Has anyone actually seen or heard of this happening?. Also, what do the Safety Codes say about the storage and operation of large banks of batteries, with regard to shrouding of terminals and ventilation of the dangerous gases that Lead-Acid cells generate?. Your comments would be most welcome!.
I don;t have much experience of such battery banks. However I know a gent who used to work in the haulage industry as a fitter. He tells of a chap ( and once showed me a picture of it ) who had been in the firing line when a large tractor battery went up in his face.
Apparently he dropped a spanner on top and shorted the studs. The resulting flash ignited the gas given off as the battery was on a heavy charge. The case exploded and showered him with the contents... Need I say more?
He lost the sight in one eye and his front teeth( actually due to 'shrapnel' ) but most of the skin on his face and chest was melted off. The photo was ghastly.
You are no doubt completely right to be wary and well protected!
If hindsight were foresight, we'd all be millionaires!
Re: Battery Bank Safety?.#150181 12/10/0406:13 PM12/10/0406:13 PM
The type of cell that the bank is constructed of makes a difference and you didn't specify which type. The most common types are flooded lead acid cells and flooded Nicad. Both are used for emergency lighting and diesel starting. A couple basic things to remember when working around battery banks regardless of type. Always follow the manufacturers instructions for isolating the bank from the charger/inverter/engine control systems before starting work. Remember that a connection that is loose will cause a tremendous arc if one of the previously mentioned devices attemps to energize,it doesn't matter if it's a charge or a discharge cycle. Never use any device that will create a spark or flame unless you have evacuated every cell and are sure that no load or charging is taking place that can create explosive gases. Ventilate the area before doing any load tests to make sure any gas that is generated is removed.Cover any adjacent cell and intercell connectors before starting work. I like to use a welders blanket as it insulates well and is tough enough for service work. Before removing any connections verify that no current is flowing. Some of these systems have DC voltages as high as 240 volts on a string so use your gloves, these strings unlike an AC derived voltage cannot be locked out. Make sure to lock out all equipment either feeding or fed from this bank. If you have a series parallel string remember to look over the installation to be sure where your connections are. Before reenergizing the equipment do a double or triple check on the tightness and correctness of any work done.All cells are marked with + and - so make sure you get them installed properly. I hope this helps with your service work I couldn't possibly cover all bases but these the basic. Just remember to work safe as you may be dealing with a lot of energy. Glasses gloves shields and insulated tools.
Re: Battery Bank Safety?.#150182 12/10/0406:27 PM12/10/0406:27 PM
I forgot one part of your question. I was once called in on a bank of cells that had not been watered properly and an internal arc occured above the level of the electrolyte. One thing led to another and during an engine start heavy currents caused multiple explosions. These cells were in a inplant substation with 12.5KV-480v transformers. The transformers were only 6 Ft. from the cells. The explosion destroyed 1.5 million worth of equipment. I feel I have been lucky as the systems that I have worked on have never had a major failure. I believe the reason is not luck but just paying attention to detail and either replacing or disabling (not always a popular move)offending parts/equipment. Good luck.
Re: Battery Bank Safety?.#150183 01/05/0508:48 AM01/05/0508:48 AM
Powercon, Welcome to ECN!. You sound like you have a good working knowledge of Secondary Cell installations. Don't get me wrong, mate, if these things only had water in them it wouldn't be an issue. However, with the type of Battery I was talking about (Lead Acid type), I don't like the idea of being showered with fragments of battery body and Sulphuric Acid, should a short-circuit occur. Which brings me to this rather dumb question. Connect 10 12 Volt DC batteries in series, to give 120V, assume that each battery is 1Ah in capacity. How would the PSCC compare to an AC supply and in fact what would the PSCC be?. Would this current cause the individual cells to explode?. And for an even sillier question, what is the best type of protection to use with a DC battery installation?.
Re: Battery Bank Safety?.#150184 01/08/0509:05 PM01/08/0509:05 PM
Trumpy, The PSCC is going to depend on a few factors and I am not trying to sidestep the question. Cells can explode due to a number of factors and I will list a few. Electrolyte to low in the cell which allows gasses to come in contact with conductors that could short or arc when heavy currents cause them to melt. Acummulated gas in a cell which is ignited from an internal falure such as a weld breaking loose from rough handling or improper fastener installation. Equipment failure causing over charging which causes excessive gasing and an external spark or flame. Lead acid cells from different manufacturers have a varying internal resistance which is the limiting factor in a PSCC, deep cycle, SLI, VRLA, they are all different. One can determine this by the formula RB = (EB - VL) / IL. Or, battery resistance = (open circuit battery voltage - specified loaded voltage)/ load current. The short circuit current (PSCC) is computed by the equation Isc = EB / RB = EBIL / (EB - VL). One point also to remember is the limiting circuit conductors. Many stationary systems are now using a type of cable limiter to prevent catastrophic breakdowns in wiring and insulation in case of a short circuit. Comparing this to an AC circuit is difficult because the AC supply doesn't diminish over time as does the battery voltage even though the instantaneous currents maybe higher for a given voltage. Also the AC supply has ocpd's in in each branch. Your question " And for an even sillier question, what is the best type of protection to use with a DC battery installation". Could best be answered by saying "As much as you can work safely in". I personally have a heavy leather apron, long gloves with leather protectors, glasses with restraints, face/head shield, insulated tools, heavy blanket for covering nearby cells and a insulating shield for working inside live panels. I personally have had primary cells explode in my face without any injuries because of what I wear. One thing to remember is never trust what the other guy did or says, if you do this usually ends up with an accident. I usually look over the installation thoroughly and try and determine what looks good and what doesn't and work backward to make everything as safe as possible. There are as many ideas about how cells should be maintained as there are about wiring a house. Also the mechanical framework and structure to support the cells should not be overlooked. Sorry if I got a little long winded but some times there just isn't a short answer.