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| External Battery for VML http://www.paulcbuff-techforum.com/viewtopic.php?f=8&t=1475 |
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| Author: | Thesepaperwings [ Tue May 24, 2011 1:17 pm ] |
| Post subject: | External Battery for VML |
Is there a reason that a 14.8 LiPo pack would not work with the VML? I have a 20AH pack and was wondering if I could connect it for extended use. |
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| Author: | Technical Support [ Tue May 24, 2011 2:32 pm ] |
| Post subject: | Re: External Battery for VML |
From the information given alone, we cannot say. The battery needs to be able to support a continuous discharge rate of 20 amperes DC. LiPo batteries tend to have a higher internal resistance, and we would suspect that it will not have a sufficient discharge rate, but do not know for sure. |
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| Author: | Thesepaperwings [ Fri May 27, 2011 4:24 pm ] |
| Post subject: | Re: External Battery for VML |
The discharge rate is a 16/32A setup. What would the drop due to the inverter. BTW my VML does last long enough but I have 100's of these packs so it would be great if I could use them. |
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| Author: | Luap [ Sat May 28, 2011 12:31 pm ] |
| Post subject: | Re: External Battery for VML |
TS is slightly off in stating LiPo batteries have high internal resistance than SLA. They are about the same when fully charged, then the SLAs develop higher and higher resistances as they discharge. This results in ever lengthening recycle times with SLAs, and non usability long before they are actually discharged. LiPos retain a nearly constant resistance all the way to the end of charge . . . thus constant fast recycle times and use of about 95% of rated AH. The batteries you suggest should work fine as long as they are rated to deliver at least 22A continuous current . . . that's the max a VML draws during recycle. |
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| Author: | Thesepaperwings [ Tue May 31, 2011 4:00 pm ] |
| Post subject: | Re: External Battery for VML |
OK, next question. This pack has been built and works but it is a different setup then I am used to so I have a question about the safety of it. The LiPo pack does not have a board on it to keep the over/under charge issues at bay. Instead it has multiple small boards on the cells pairs. Is this ok? normally I would use a joiner and use a small board to control it all but these are already done so I'd like to see if they will work. |
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| Author: | Luap [ Wed Jun 01, 2011 1:07 am ] |
| Post subject: | Re: External Battery for VML |
What you describe is called a balancing charger. This is standard technology for RC toys. Provides very low internal resistance, but no output current protection. Should work OK with VML, but be careful . . . if something goes wrong or if you short the output this type of battery can explode, overheat or go up in flames. I would mount the batteries in a metal box of some sort for safety. |
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| Author: | Thesepaperwings [ Wed Jun 01, 2011 11:11 am ] |
| Post subject: | Re: External Battery for VML |
That's what it is. It's a pack made from smaller RC setups. We used it for voltage testing. I was thinking that if I pot them in a pelican case there would be no reason for a short. Prototype packs that are set this way are good for a 3 foot drop test. What do you think? |
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| Author: | Luap [ Wed Jun 01, 2011 11:57 am ] |
| Post subject: | Re: External Battery for VML |
VML has a 30A fuse between battery and inverter. We have never had a failure of inverter or battery. But there is always a possibility of a short somewhere in the wiring to the VML. RC packs are notorious for going up in flames. Make sure it's rated at 30A continuous output. |
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| Author: | Thesepaperwings [ Wed Jun 01, 2011 3:39 pm ] |
| Post subject: | Re: External Battery for VML |
OK, looking at this pack here is what I found. It is a 20 cell pack. The cells are 7.4V 2,000Mah with an 8C rating. They are tabbed in parallel blocks of 10 that make it 7.4V at 20Ah. The two blocks are wired in series to produce a 14.8V 20Ah pack but what does that make the amp rating? I thought discharge rate did not change while making packs but It's a little blurry. |
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| Author: | Luap [ Thu Jun 02, 2011 3:06 am ] |
| Post subject: | Re: External Battery for VML |
2000mAH is 2 AH. 8C means continuous current is 16A. If there are 10 cells in parallel, this can deliver 160A. So 10 parallel x 2 series gives 160A at 14.8V. VML only draws a max of 23A so you should have no problems. Must be mighty big packs! |
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