Thanks Frank. What I don't know about batteries would fill a book. Unfortunately it would be a book called "Things you really need to know about batteries"...
I've found an SLA 12V/18ah (http://www.technobotsonline.com/haze-hzb- ev-sla-agm-ev-battery-12v-18ah.html) which says: Capacity based on 20 hour discharge 17Ah Capacity based on 1 hour discharge 11Ah Cold cranking current @ 20 Deg C 214A Recommended maximum charging current 3.4A Reserve capacity for a 20A discharge current is 27 minutes I looked up the 12V7ah one, and you were right, based on a 1 hour discharge the amp-hours went down to 4.2ah. Now I'll have to hunt and find soe Ni-Cads... Thanks again. Phil On May 31, 1:49 pm, Frank Pittelli <[email protected]> wrote: > Your math is right, but not your physics. You can't use 100% of an SLA > battery's capacity ... you can barely use 50%. At 20 amps, the 7AH > battery plates are working very hard to push electrons and are being > pushed into inefficient operation. The inefficiency shows up as heat. > If you use larger batteries (say 16Ah), not only will the capacity > increase, but so will the efficiency, because the batteries have larger > plate surface areas. Basically, you're trying to suck molasses through > a narrow straw ... get a bigger straw. > > If you switch from SLA batteries, to another type, such as NiCad, you'll > also see better performance. NiCad batteries can deliver almost 100% of > their capacity and, better still, they can deliver current at higher > levels. In fact, back when we ran model warships, we determined that an > SLA battery of X AH capacity could be effectively replaced by a NiCad > battery of X/2 AH capacity. Given that the density of NiCads is less > than SLAs, that's a double weight savings. NiCads do cost more, but > they are worth the extra expense in the long run. > > There is also a new breed of SLAs coming on the market which use better > anodes to deliver almost the same performance levels as NiCads, while > keeping the cost close to standard SLAs. I haven't seen them sold in > the mass market yet, but they're definitely on their way. > > Frank P. > > On 5/31/2011 8:31 AM, Phil wrote: > > > > > I've been carrying out some tests on the Hetzer T057. I couldn't work > > out why it would run for a short period then stop. I think it might > > bge overheating batteries,. But do batteries overheat?I need some > > advice on how to extend power. > > > I carried out some tests over the weekend. The Hetzer is powered by 2 > > no. 18V de Walt drill motors. Witreasonably charged-up 6V and 12V > > batteriies (7.2Ah and 7Ah respectively) connected in series the thing > > managed 4 laps of the garden (ie about 195 yards in total) at a lively > > 3-4 mph, slowing down on the last lap, at which point it stopped due > > to lack of power. Motors were lightly warm, but batteries were > > noticeably warm, like a hand-warmer. I left them for 10 mins or so and > > then we got another 65 yard lap, but which time it was slowing down. > > It would then do the same thing- 10 mins break followed by 1 lap- for > > another couple of laps or so, by which time voltage was down under 17V > > and the batteries were just flat. > > > I believe these motors are about 180W each, which at 18V means they > > are drawing some 20 amps (there is no speed controller so the motor is > > either "on" or "off"). Certainly the main fuse (30A) has never blown. > > That means that I should get some 20 mins run time, or probably about > > 40 mins battling, as you're not moving all the time. In fact I'm > > getting about 5-7 mins, though there's a lot of friction in the tracks > > so I may "lose" the later part. > > > Can anyone advise on what I ought to do? When battling the tank should > > stay cooler inside as the batteries are right next to the gas bottle, > > which was warmed by the battieries during my test, but would be cold > > when fired and therefore a cooling element in itself. Will cooling > > help? I can try some experiments like using ice packs or leaving the > > top off (to simulate adding a fan and cooling vents) but I'm not sure > > if warm barrieries are (a) normal (b) mean defective batteries or (c) > > are the probable cause of the problem. > > > Thanks guys > > > Phil- Hide quoted text - > > - Show quoted text - -- You are currently subscribed to the "R/C Tank Combat" group. To post a message, send email to [email protected] To unsubscribe, send email to [email protected] Visit the group at http://groups.google.com/group/rctankcombat
