At my workplace we have several emergency lighting fixtures (the kind that will run on their own battery within each, in the event of a power outage). The batteries are this type: http://i207.photobucket.com/albums/b...ps5164ff8c.jpg
These fixtures have trouble indicator lights and beeps that will happen in varying sequences depending on the problem. A few of them have recently begun indicating "battery capacity failure" which typically means the battery needs replacing. So I removed the batteries and out curiosity checked each with a multimeter and checked voltage readings. I got at least 6 volts from each battery. I do know these batteries are relatively old by now, probably 4-5 years anyway, and the fact that I'm getting a low battery indicator does not surprise me, but I can't understand why they still check out at 6-plus volts. Comments/explanations? Thanks.

 

6v 12ah is a big fixture. Most emergency lighting fixtures use a 6v 4ah lead acid cell.

If your fixtures beep then they must have a test circuit that checks the battery under load. A charged 6v lead acid battery should check at upwards of around +/- 6.5v.

Wait until you go to buy that battery. You'd better bring a chair to sit down on.

 

Not sure what you mean, PJ, "a test circuit that checks the battery under load". And yes when I checked these batteries they checked around 6.5v as you mentioned. Yeah I know these batteries are very expensive to replace; I'm not sure why we have to have these kind of emergency fixtures that use such expensive batteries.

 

Expensive emergency fixtures......just like most alarm panels..... have a circuit that disconnects the battery from the device and applies a load to it. It measures how fast the voltage drops and if it drops below a certain preset value..... it determines that the battery will no longer hold the charge that it was designed to.

Hence it announces ...... check or replace battery.

 

So without having to understand much about the actual mathematical explanation of what exactly the meaning of 12 amp-hours is, I guess when I get the check or replace battery indicator light/beep, it means the battery will no longer hold the charge that could operate the emergency lamp(s) for a particular amount of time, such as maybe 30 or 45 minutes or something? Maybe if I thought a shorter period of time for the emergency lighting to operate was adequate, I could substitute the 12ah battery for a 4ah, as a measure of saving money on battery replacement? Or is the 12ah fixture designed specifically for a 12ah battery only, and if I wanted to be able to use 4ah batteries I'd need different (new) fixtures?

 

You could substitute a smaller AH battery for a shorter run time. I purchase 6v 4ah batteries by the case. What I would do would be to put in at least 2) 4ah batteries in parallel.

 

That's exactly what it means. Simply put, a 12Ah battery can supply 12 amps for one hour or one amp for 12 hours (or anything along that graph).

Fire code requires emergency egress lighting to function for a minimum of 90 minutes. Given that those fixtures use 12Ah batteries, they most have high-power bulbs in the heads. They likely also feed remote heads. Installing the 4Ah battery cuts the runtime by 1/3, which will make them non-compliant with fire code. Also, since these fixtures have capacity monitoring circuits, it is very likely that using a 4Ah battery will cause it to indicate failure.

You must use the 12Ah batteries.

 

JerseyMatt,
Thanks. That's kind of what I figured, that there's valid reasons I'm (well, my workplace anyway) is probably pretty much stuck with having to replace these expensive batteries. The one thing you mentioned, though, that they likely also feed remote heads, does not happen to be the case with the ones we have. They just feed the bulbs in the heads.

 

There's no head outside near a door that is fed off of one of the inside wall-packs? OK.

This might be a good time to replace them.

 

Yeah I know it's apparently time to replace them; I just was unclear on why when I checked with the multimeter the voltage still seemed fine, but I guess I understand now that doesn't really tell the whole story. Thanks all.

 

The 'open circuit' voltage (sticking a multimeter on the terminals with nothing else connected) really doesnt tell you anything about a battery's condition. Basically you are reading what is called a 'surface charge'. It's the same reason a simple multimeter wont tell you if your car battery is good or not. Batteries must be tested under load - and that's what the monitoring circuit in the lights does. I reads the voltage and rate of discharge WHILE the lights are activated so the battery is under load.

 

I see. I've been kinda clueless about that all along. How about checking something like a regular flashlight battery, say a C cell or whatever, with a multimeter? If I really want an accurate test of it I'd need to check it under load? Also, I was wondering about the "rechargeable" aspect of my emergency light batteries. Says they're rechargeable right there on the battery. So if I had a charger I could maybe charge 'em up with the charger to get more life out 'em, before replacing?

 

Your emergency light fixtures should have built-in trickle charging for the batteries. check the info that came with them.

 

Yeah the lights charge the batteries constantly while they have power. Basically they are sealed lead acid (SLA) batteries, similar to but not quite the same as the kind in your car. Unlike NiCd/NiMH rechargeables you would use in your digital camera or flashlight, SLA's do not develop a 'memory' effect and like to remain fully topped off. However, even when perfectly cared for, SLA's do degrade over time. There is nothing that can reverse that process. It simply destroys the electrode material to the point where it can't hold a charge anymore.

 

Acutally, the info that came with them is gone (lost or misplaced, never kept around), but I'm sure still available from the manufacturer if I'm so inclined to acquire. But okay I get it now, the fixtures have the built-in trickle charge feature/function that does the recharging apparently. thanks Nashkat

 

Yes, of course, your emergency light has it's own charging circuit. Otherwise the battery would have went dead years ago.

A lead acid battery........ constantly under charge ......can only last so long. The charging circuit bakes the battery. Lead acid cells in emergency applications, as well as for alarm systems, are rated for three to five years.

There was a time you could get 5,6,7 or more years on lead acid batteries. Those days are gone. Most lead acids for cyclic use are

 

I thank you guys for all the the explanations. Cleared things up for me about those dang emergency light batteries.

 

I actually missed that you missed the point of what Nashkat was saying when I replied.. Double whiff!

He was saying that given the cost of the batteries, you may be better off looking into replacing the whole fixtures at this point. There have been major advances in lighting tech over the past few years, and for the same or a bit more than what you'd pay for a replacement battery, you might be able to replace the entire fixture for one that maybe uses high power LEDs and a smaller, cheaper battery. So next time the battery goes it won't be such a wallet buster.

 

Oh okay, I see, yeah I did miss that point of Nashkat's. Okay it's time for me to go shopping for some updated replacements for those old emergency light fixtures. Thanks. Will do!

 

Only under load would you get an accurate test of other kinds of batteries such as C cells also.

But each time you do a load test you use up some of the battery.

Emergency light sets that come with rechargeable batteries usually have a built in charger.

 

Gotcha. Thank you AllanJ

 

Have you checked the price of replacement batteries yet? As I recall, the price is not as bad as you may be thinking. I remember that most exit/emergency combination batteries run between about $7 and $20, depending on which battery and where you buy them. A good electrical supply house should have a variety of replacement batteries or you could go to a store like Batteries Plus, but they would be a little more expensive there. Check it out.

PS-6100-F1 Power Sonic Replacement SLA Battery (6.00 Volts 12.00 AmpH) - High quality and lowest price guaranteed

 

That is actually a very good price on that battery. Just have to see what the shipping would do to the price.

 

Yeah I guess the price isn't all that extremely bad on those batteries, but the thing is my location, which happens to be in a relatively remote SE Alaska community (no such batteries in stock anywhere locally) where shipping costs, especially for these relatively heavy items is often a cost-prohibitive factor. Anyway, we went ahead and made the decision to purchase the more modern type fixture(s), such as described by JerseyMatt (post #18) to replace what we have. We were able to get these for $66 each: AP Series with Heads

 

Yep, those are adequate and the exit sign also has LEDs, no bulbs to change.

 

Another option available to us is as shown in the following link. The choice between this one and the other one (link in post #24 here) is that the Compass brand model uses a nickel cadmium battery and the other one uses a "lead calcium" battery. Compass Lighting Any comments appreciated to help me decide between the two. Thanks again.

 

I don't think it makes much difference. Either one will give you good service.

 

That's good to hear. I think the nickel cadmium battery(s) are lighter in weight and less bulky than the lead calcium type, which would mean less costly to ship here when it comes time to replace them.