Control/Dimming Testing batteries for Wireless Set Pieces

TheTheaterGeek

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Im having issues testing the batteries for our wireless set pieces in the current show I'm doing. I have inverters connected to a 12 v 35ah battery. One inverter is 1500w and one is 750w just based on load need.

I multimeter the batteries and get around 12.5 volts but the battery charger says it's only a quarter charged. What do i need to do in order to test the juice in the battery. With a rather basic multimeter. (no min max or other advanced functions.)
 
Voltage alone will never tell you the charge state or condition of the battery. A regular 12 v car battery tops out at about 14 to 14.5 volts. Take the charger off of it and come back an hour later and it will have dropped a bit. Put a load on it and it will drop further. All these things are normal. The only way to "test" a battery is to put a good heavy load on it and come back every few minutes and make a read. The faster it discharges and charges, the weaker the battery is. (My favorite test load is a 1k 120 volt par lamp) If you are sure you have a solid battery then the best way to prep it for stage use is to charge it up on a regular 4 to 10 amp battery charger until it is at about 13 volts. At that point, connect it to a float charger (sometimes called a "battery maintainer" at auto parts stores) and leave it connected overnight. Never leave the load on to the extent that you drop to less than 8 volts or you could do some damage to the battery! Here's another thing to keep in mind- Almost all the wear and tear on a battery happens in two ranges: When the battery is 80 to 100% charged, and when the battery is less than 40% charged. Keep it between 40 and 80% and the battery will be around for many many years.
The only true read on the charge state of a battery is the ph of the acid in it. Kind of hard to read on modern sealed batteries and gel cells.
 
Voltage alone will never tell you the charge state or condition of the battery. A regular 12 v car battery tops out at about 14 to 14.5 volts. Take the charger off of it and come back an hour later and it will have dropped a bit. Put a load on it and it will drop further. All these things are normal. The only way to "test" a battery is to put a good heavy load on it and come back every few minutes and make a read. The faster it discharges and charges, the weaker the battery is. (My favorite test load is a 1k 120 volt par lamp) If you are sure you have a solid battery then the best way to prep it for stage use is to charge it up on a regular 4 to 10 amp battery charger until it is at about 13 volts. At that point, connect it to a float charger (sometimes called a "battery maintainer" at auto parts stores) and leave it connected overnight.

The only true read on the charge state of a battery is the ph of the acid in it. Kind of hard to read on modern sealed batteries and gel cells.

I figured that voltage alone cant be a solid indicator. How would the battery charger know when the battery is full? Is there a way to replicate that process with a DMM?

Also, Thank you for the great description.
 
A standard battery charger doesn't know and can overcharge the battery. A smart charger 'assumes' the battery is solid and floats it up to the full charge voltage, which depending on the manufacturer, can be 13.8 to 14.2 volts. A smart charger will also time the voltage increase and throw an error if the battery rises too fast. Battery tenders actually modulate the voltage, trickle charging then discharging slightly to keep the battery floating near full charge but exercise it in the process. The problem is that all chargers try to keep the battery fully charged which actually can shorten it's life.
I have an ongoing experiment with my wife's old cell phone. (2007) She has never charged it completely nor let it discharge completely. So far, we have 8 years on that battery.
 
A standard battery charger doesn't know and can overcharge the battery. A smart charger 'assumes' the battery is solid and floats it up to the full charge voltage, which depending on the manufacturer, can be 13.8 to 14.2 volts. A smart charger will also time the voltage increase and throw an error if the battery rises too fast. Battery tenders actually modulate the voltage, trickle charging then discharging slightly to keep the battery floating near full charge but exercise it in the process. The problem is that all chargers try to keep the battery fully charged which actually can shorten it's life.
I have an ongoing experiment with my wife's old cell phone. (2007) She has never charged it completely nor let it discharge completely. So far, we have 8 years on that battery.

Thats interesting. I have always thought it would be so much easier.

8 years? Thats insane. What is the experiment?
 
8 years? Thats insane. What is the experiment?
As I had mentioned in Post #2, you can get some pretty long life out of a battery by not charging over 80% or discharging below 40%. The (very casual) experiment is to see just how many years we can get out of a battery that has an expected life of about 2 years by keeping it in that range. 8 and counting. I am making the assumption on that based on each bar representing about 20% charge (there are 5), so we charge when we hit 2, and stop charging at 4. Very unscientific, I know, but it still holds its charge level well in use which tells me the battery is holding up pretty well.
 
As I had mentioned in Post #2, you can get some pretty long life out of a battery by not charging over 80% or discharging below 40%. The (very casual) experiment is to see just how many years we can get out of a battery that has an expected life of about 2 years by keeping it in that range. 8 and counting. I am making the assumption on that based on each bar representing about 20% charge (there are 5), so we charge when we hit 2, and stop charging at 4. Very unscientific, I know, but it still holds its charge level well in use which tells me the battery is holding up pretty well.
40-80% is a great range to keep lithium based batteries, my understanding is that lead acid batteries should be kept closer to full as much as possible.
 
40-80% is a great range to keep lithium based batteries, my understanding is that lead acid batteries should be kept closer to full as much as possible.

This is somewhat my understanding as well. Hence why car, boat and fire alarm and security systems are always at full charge and use lead-acid instead of other kinds of batteries.

Oh how I remember using an somewhat antiquated charger for our 5 boat batteries for over winter, checking the acid, etc when I was a kid. I wonder how much I can vaguely remember as I haven't dealt with batteries in YEARS.
Changing between Constant-current charge, topping charge, and float charge. Most modern systems will do this automatically.

You don't charge to a %, you charge by way of a desired or recommended float charge. As you charge the battery the voltage will increase, but really charge is measured in Ah (Amp-hours) if I recall correctly. Of course Ah is not really directly measurable, so we go by Voltage (and in non-sealed cells by the fluid can be measured to help maintain the battery).

Basically my point is yes, if you "overcharge" your battery it will fail much quicker. However if you maintain a proper charge level it will last a good long time, hence why setting a proper float charge was important in my garage as a kid.
 
This is opening up a lot of electrical curiosities for me. How do those AAs with the "press here and here to see the charge level" work?
 
This is opening up a lot of electrical curiosities for me. How do those AAs with the "press here and here to see the charge level" work?
Basically, a resistance strip and some temperature sensitive liquid crystal. Push and it makes contact and heats up the strip. Not all that accurate, but very handy!

As for lead acid batteries, not sure about that 100% thing. Gel-cells are basically lead acid but gelled and they tend to cook out in about 3 or 4 years from being kept at 100%. Batteries tend to dry out when kept near 100% even lead acid. If you are able to keep them watered your in good shape but many modern batteries don't give you that option. Overcharge kills them for sure. Somewhere I have a chart of a study on battery life, and most all batteries have a happy spot! It is impractical to not fully charge some batteries. Who wants to know they only have a 60 or 80% charge on their car battery on a cold winter's morning? What happen at the discharge end can wreck a battery even faster. You can actually change the chemistry of a battery near full discharge and make it inert. Also, a battery with a low charge may actually reverse-polarity it's weakest cell when put under load which can have catastrophic results.
 

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