fly boy3 Posted October 4, 2015 Share Posted October 4, 2015 For many years I have been using a battery checker for my Rx battery. It is the type that puts a load on same, with coloured leds, indicating high, med and low. Just thought that what difference would it make on different batts, say 1000Mah and 2000Mah, as I presume the same load is put on both batteries. Cheers Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted October 4, 2015 Share Posted October 4, 2015 Shouldn't make any difference FB3......by applying a resistive load the checker will cause a certain current to flow....a battery in good nick should supply the current without undue volt drop weather it's 1000mAh or 2000mAh. A battery in a poor state or close to discharged would show significant volt drop & this is your "stop flying" warning. That said though in the case of AA cells, cramming every higher capacity into the same package means that something has to give. Higher capacity AA cells have much thinner plates (the only way to fit the greater capacity plates into the case) & these thinner plates can mean that they cells are more reluctant to supply higher currents than cells of lower capacity which have thicker plates.... Quote Link to comment Share on other sites More sharing options...
fly boy3 Posted October 4, 2015 Author Share Posted October 4, 2015 Cheers Steve got it. So are we saying the leds are showing the voltage drop across the battery after a load has been applied ? Cheers Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted October 4, 2015 Share Posted October 4, 2015 That's usually the idea...!! Even a nearly knackered battery will often show full or close to full voltage. Apply a load to it & good battery will hold it's voltage up OK whilst the voltage of a dodgy battery will simply melt away... Quote Link to comment Share on other sites More sharing options...
fly boy3 Posted October 5, 2015 Author Share Posted October 5, 2015 Thanks Steve. Cheers Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted October 5, 2015 Share Posted October 5, 2015 Often worth wiggling the servos when it's connected, if its fully charged and the checker drops into the red then you may be on thin ice and further checks are required. Apart from batteries this check is worth doing to check on the condition of a switch, I had the Rx battery voltage (measured at the Rx) drop when moving the servos, turned out the new switch was faulty and had a big voltage drop across it when any real current flowed. Quote Link to comment Share on other sites More sharing options...
SIMON CRAGG Posted October 5, 2015 Share Posted October 5, 2015 We are starting to explore the benefits of LiFepo4 2 cell rx packs, and use a modified multimeter to check pack voltage which coloured lights do not reflect enough detail on the state of the pack. We would never go back to NIMH! Quote Link to comment Share on other sites More sharing options...
Geoff S Posted October 5, 2015 Share Posted October 5, 2015 A battery can be represented by an ideal, perfect battery with a resistance in series with it. So, if that resistance has a value of 1 ohm then drawing a current of 1 amp will result in a voltage drop of 1 volt - quite a lot if the battery is only a 5 volt one! In practice of course a 1 ohm series resistance is very high and a more likely value is 5 or 10 milliohms. As the ideal battery voltage falls during its discharge cycle then the voltage drop across the effective series resistance (esr) becomes much more significant and an on load check gives a good warning of not to fly! It isn't helped that the esr also increses slightly as the battery discharges. The capacity of the battery has no effect on the onload test. Except, as mentioned above, the higher capacity batteries tend to have a higher esr because of their construction and there deliver less current for a given voltage drop. Geoff Quote Link to comment Share on other sites More sharing options...
Martin McIntosh Posted October 5, 2015 Share Posted October 5, 2015 I don`t know the make of checker you are using, but a club mate recently asked me to look at his which was obviously telling him porkies. The calibration of the device was done with a single pot. which was very sensitive and had wandered off after time. The only answer is to check the checker with a calibrated DVM occasionally. The current crop of these from Hobbyking do not apply a load and state that they are for a rough indication only on Nimh cells but seem to be quite accurate from my experience. You could always add a suitable parallel resistor to simulate a load. I tend to go by a typical flight on normal servos using about 110 mA/hr. Quote Link to comment Share on other sites More sharing options...
SIMON CRAGG Posted October 6, 2015 Share Posted October 6, 2015 We have been carrying out a lot of testing on Lifepo4 rx packs over the last few weeks. A five servo (not digital) servo model is using about 55mah per 10 min flight + a couple of minutes added on to get to the flight line etc. Timer started from TX switch on. More trials on models with retracts / flaps etc in the pipeline. Only using a max. of 60% / 6.4v of total cell capacity prior to charge. Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted October 6, 2015 Share Posted October 6, 2015 I've been using a 2100 mah 2s LiFe battery in my 1:3.5 scale Topaze (2 x aileron, rudder, elevator, 2 x spoilers and tow release servos), full day at an aerotowing meet, 6-8 flights each 5 to 40 mins in duration (subject to thermals) and at the end of the day it takes less than 1100 mah to top it up. After the initial dip the voltage remains very stable, hardly fluctuating under load. But the flat discharge curve means that battery checkers which give you % left struggle, my one always under estimates how much is left. Quote Link to comment Share on other sites More sharing options...
SIMON CRAGG Posted October 6, 2015 Share Posted October 6, 2015 Agreed. Best bet is to rig up a multimeter and suitable plug / socket on the side of the fuz. Check voltage regularly ( stir the sticks at the same time), and monitor the voltage. 6.4V = about 40% left in the pack. As with most things battery...........hundreds of different opinions! Quote Link to comment Share on other sites More sharing options...
fly boy3 Posted October 6, 2015 Author Share Posted October 6, 2015 Thanks guys, learning a lot about batteries now. As a 2cell Life battery equates to 6.4 v, would this have an effect on the servos, Rx etc ? Cheers Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted October 6, 2015 Share Posted October 6, 2015 Actually 6.6 volts nominal FB3 & 7.2V hot off the charger but this soon drops to 6.6V. Will it have an effect....well most 4 cell NiMH packs are close to 6V when fully charged & a 5 cell NiMH pack will be in excess of 7V so a 2 cell LiFe pack should be fine. I've been using them for years with no problems at all. Be aware that they have a pretty flat discharge curve & will hold around 6.6V until nearly flat & then the voltage just melts away. HK do a range of battery monitors including one for lithium based batteries (has a LiFE & LiPo setting) & I've found these to be very good. Fit one & get to know it & don't fly if you are anywhere near the red LEDs The big advantage of LiFE is the easy charging regime...charged up in less than 1 hour..with no false peaks like you get with NiMH batteries plus the fact that they hold their charge for weeks (unlike NiMH which go flat while you look at them). Charge them when you get back from the field & fly again weeks or even months later. Their lighter weight is just a bonus. I've been using the HK 1100 mAh packs & these are fine for "sport use" easily enough for a full afternoons flying Buy with confidence I say but do make sure your charger has a LiFe setting...these batteries are different to LiPos. Quote Link to comment Share on other sites More sharing options...
ChrisB Posted October 6, 2015 Share Posted October 6, 2015 We've been using the LED checker strips for some time, but were never convinced that they were that acurate and not sure if they give a load. I think they just monitor the load. So, we bought one of the Futaba checkers which will load at anything between 1 and 3 amps for 4.8, 6 and 7.2 v. Although not cheap, it shows the load and volts on a screen. Obviously not 100% acurate, but at 2 amps a 4cell 3300 sub c pack was showing 5.5v, when the LED checker strip was showing all red. Quote Link to comment Share on other sites More sharing options...
SIMON CRAGG Posted October 7, 2015 Share Posted October 7, 2015 I am not a great believer in these light strips either, as I have found they differ wildly and what does each light reflect exactly?. The only way to check Life rx cells is with a multimeter IMHO. You get a decent 2 decimal point readout, and they can easily be calibrated if you know an electrician!. They drop like stone from 7.2v to 6.6v and then hold 5.5 for hours. Good bit of kit. Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted October 7, 2015 Share Posted October 7, 2015 Posted by SIMON CRAGG on 06/10/2015 19:16:20: Agreed. Best bet is to rig up a multimeter and suitable plug / socket on the side of the fuz. Check voltage regularly ( stir the sticks at the same time), and monitor the voltage. 6.4V = about 40% left in the pack. As with most things battery...........hundreds of different opinions! Or if you have telemetry read the voltage off the Tx I have my alarm set at 6v on my 2S LiFe batteries and it's never yet gone off, but if it does it will be time to land pronto. Quote Link to comment Share on other sites More sharing options...
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