Post by thread:[EE]: Automotive battery capacity

I need to power some electronics at a trade show for 3 days (total of 26 show hours) and won't have an AC outlet. I need to calculate how many automotive batteries it would take to power these electronics. As a start, the good news is that the electronics are designed to run off automotive 12V -- and have been tested down to 8.5V. Altogether, I estimate 20W consumption (mostly DC-DC converters, so that power should be fairly consistent as battery voltage decreases). Now, I have a battery here that claims 58AH. My google research tells me this means 14.5A for 4hrs, at which time the battery voltage will be 10.5V. But that is apparently 10.5V *loaded*. So what happens if I only consume 2A -- does it also mean that I can get ~2A (being conservative) for 29 hrs and the ending voltage will still be 10.5V? I doubt since the load is different. Generally a car battery is considered 0% charged when the open-circuit voltage drops to 11.8V. And generally considered to need charging (75% charged) when the voltage drops below 12.4V. But that's for a car -- and I'm sure there's a lot of useable capacity below that. So what happens below 11.8V open-circuit? Is the discharge (fairly) linear? I could not find a battery discharge graph via google. What is the minimum safe voltage to charge a battery? Bottomline, I'd like to know if one fully-charged battery will be more than adequate for all 3 days. If absolutely necessary, I'll drag it back to the motel and charge it, but really want to avoid that -- something about a lead-acid battery in an enclosed room (that I'm sleeping in) that does not thrill me. Can any of you lend some intelligence with this? Thanks, -Neil.

PicDude writes: > I need to power some electronics at a trade show for 3 days (total of 26 show > hours) and won't have an AC outlet. I need to calculate how many automotive > batteries it would take to power these electronics. First off, you do *not* want to use an automotive (or starting) battery. Automotive batteries are designed to provide a lot of current for a short period of time, and are not designed to be drained more than 10 or 20% from full capacity. Draining an automotive starting battery to empty is a good way to kill it, or at least reduce it's capacity dramatically. Instead what you really want is a deep cycle battery -- the kind used in golf carts, or as the "house batteries" in sailboats and RVs. Off-the-cuff, your tech info seems ballpark, but be sure to check out the car and deep cycle battery FAQ: http://www.uuhome.de/william.darden/carfaq.htm > Bottomline, I'd like to know if one fully-charged battery will be more than > adequate for all 3 days. If absolutely necessary, I'll drag it back to the > motel and charge it, but really want to avoid that -- something about a > lead-acid battery in an enclosed room (that I'm sleeping in) that does not > thrill me. Deep cycle batteries come in all different sizes. If you underestimate, you can always pick up an additional battery. You will find quite a bit of difference in price WRT deep-cycle batteries depending on construction. There are several types of sealed batteries that offer many advantages which you may or may not need. Properly cared for, though, the cheapest golf cart battery will often last just as long (overall lifespan and total number of recharges) as sealed battery costing two or three times as much. -p.

On Monday 07 February 2005 12:20 am, Peter Johansson scribbled: > First off, you do *not* want to use an automotive (or starting) > battery. Automotive batteries are designed to provide a lot of > ... > > Off-the-cuff, your tech info seems ballpark, but be sure to check out > the car and deep cycle battery FAQ: > > http://www.uuhome.de/william.darden/carfaq.htm Ah-ha! I did not know the difference between regular and deep-cycle batteries. But your mention of them now helped me also locate other useful info, and I should have access to (borrow) an Optima deep-cycle battery. What's curious/interesting though is that the FAQ you link to mentions in #1.7 that a regular battery should not be discharged below ~12.0V, but #1.12 says that a deep cycle battery should also not be discharged below ~12.0V. Huh? So where's the advantage of being able to be deep discharged? It also mentions that shallow discharges should be avoided for deep-cycle batteries, but I will be discharging at low rates frequently. I have a voltmeter with an alarm that I can setup to alert me when the voltage level gets to the critical point. But I've still not found a clear enough answer that tells me how much useful Amp-Hours I can get off a deep-cycle battery without damaging it, and what that critical level actually is. Cheers, -Neil.

Car batteries are designed to provide a LOT of current for a short period of time, then be recharged by the alternator and kept that way. Deep cycle batteries are designed to provide a moderate amount of current for a long time, then be recharged. The internal construction is quite different. A deep cycle battery can be discharged to the 50% capacity level and then recharged several hundred times during it's life. A car battery would perhaps last a few dozen times under these circumstances. A deep cycle battery can be discharged 80 or 90% safely; doing this regularly merely shortens the lifetime. At 50% capacity, a battery will read about 12.1 volts open circuit, AFTER RESTING FOR A WHILE (several hours). When it has only 20% left, it will read about 11.66 volts (http://www.trojan-battery.com/customercare_batterymaint4.html) At 02:47 AM 2/7/2005 -0600, you wrote: {Quote hidden}>On Monday 07 February 2005 12:20 am, Peter Johansson scribbled: > > First off, you do *not* want to use an automotive (or starting) > > battery. Automotive batteries are designed to provide a lot of > > ... > > > > Off-the-cuff, your tech info seems ballpark, but be sure to check out > > the car and deep cycle battery FAQ: > > > > http://www.uuhome.de/william.darden/carfaq.htm > >Ah-ha! I did not know the difference between regular and deep-cycle >batteries. >But your mention of them now helped me also locate other useful info, and I >should have access to (borrow) an Optima deep-cycle battery. > >What's curious/interesting though is that the FAQ you link to mentions in >#1.7 >that a regular battery should not be discharged below ~12.0V, but #1.12 says >that a deep cycle battery should also not be discharged below ~12.0V. Huh? >So where's the advantage of being able to be deep discharged? It also >mentions that shallow discharges should be avoided for deep-cycle batteries, >but I will be discharging at low rates frequently. > >I have a voltmeter with an alarm that I can setup to alert me when the >voltage >level gets to the critical point. But I've still not found a clear enough >answer that tells me how much useful Amp-Hours I can get off a deep-cycle >battery without damaging it, and what that critical level actually is. > >Cheers, >-Neil. > > >

Try & get the datasheet for the battery you are going to use. Typically, a deep cycle battery will be easier to finfd the data for as the info is generally required when sel;ecting the battery (rather than finding one that fits in the space available!) The charts normally include both constant current and constant power discharge curves. The actual capacity will vary with current/power level as well as the battery construction. Your 58AHr is probably rated at the 10hour rate. i.e. 5.8A for 10 hours. At 58A you will not get an hours discharge time. The battery "endpoint voltage" is also discharge rate dependent. At the higher discharge rates it is permissable to drop to a lower voltage - 1.6 volts per cell not being uncommon in UPS applications. In contrast at lower discharge rates, the endpoint may be as high as 1.8V/cell. Once the battery endpoint voltage is reached, there is little energy left - the voltage falls off quickly and some cells may "reverse" - charge up with negative polarity. You do not want this. Recharge voltage / current depends on battery construction - again the data sheet will give guidlines. A current limited charger set to ~15V should be OK for a quick recharge but once the battery is 80-90% charged you should drop the voltage back to 13.6 or so. The current limit should be set to no more than than the 1hour rate (58A) but your charger isn't too likely to handle this anyway! If you can monitor the battery temperature then this is even better as it is possible for batteries to thermally "run away" - due to the terminal voltage decreasing with increasing temperatures. If you overcharge the battery for an extended period it will loose water and capacity. A lot depends on battery construction so the data sheet is going to be your best guide. Hope this helps. Richard P I need to power some electronics at a trade show for 3 days (total of 26 show hours) and won't have an AC outlet. I need to calculate how many automotive batteries it would take to power these electronics. As a start, the good news is that the electronics are designed to run off automotive 12V -- and have been tested down to 8.5V. Altogether, I estimate 20W consumption (mostly DC-DC converters, so that power should be fairly consistent as battery voltage decreases). Now, I have a battery here that claims 58AH. My google research tells me this means 14.5A for 4hrs, at which time the battery voltage will be 10.5V. But that is apparently 10.5V *loaded*. So what happens if I only consume 2A -- does it also mean that I can get ~2A (being conservative) for 29 hrs and the ending voltage will still be 10.5V? I doubt since the load is different. Generally a car battery is considered 0% charged when the open-circuit voltage drops to 11.8V. And generally considered to need charging (75% charged) when the voltage drops below 12.4V. But that's for a car -- and I'm sure there's a lot of useable capacity below that. So what happens below 11.8V open-circuit? Is the discharge (fairly) linear? I could not find a battery discharge graph via google. What is the minimum safe voltage to charge a battery? Bottomline, I'd like to know if one fully-charged battery will be more than adequate for all 3 days. If absolutely necessary, I'll drag it back to the motel and charge it, but really want to avoid that -- something about a lead-acid battery in an enclosed room (that I'm sleeping in) that does not thrill me. Can any of you lend some intelligence with this? Thanks, -Neil.