Parallel Batteries?

[Rick DevonshireParticipant @rickdevonshire]

I have noticed over the last couple of years a number of boat building articles have mention the craft are carrying two power batteries. ie. two 7.2volt packs. This is particularly so when it comes to beamy boats such as life boats. But there is often no mention of how they are connected. At first sight connecting in parallel should obviously double up the capacity available, lengthening time sailing between charges.

There is however a danger in this practice since if one battery pack discharges at a faster rate than the other the stronger battery may then discharge through the weaker possibly damaging it as this reverse current may then consitute a dead short.

I seem to remember some time ago seeing an article on the use of suitable “power” diodes able to withstand hight currents, being used in parallel battery configurations to guard against this happening. I can’t however remember where it was. Can anyone enlighten me please? Rick.

 

 

[Rick DevonshireParticipant @rickdevonshire]

How can two 7.2volt power batteries be SAFELY wired in parallel?

[TelstarParticipant @telstar]

Hi Rick

Installing battery packs in parallel is not in itself hazardous, if once they have been set up in parallel you leave them in parallel, and use and charge them as one battery.

ie connecting 2x2000mAh 7.2v  in parallel should be charged together as one 4000mAh 7.2v battery.

As long as they are connected together they cannot get out of balance giving circulating currents.

Problems arise when the seperate batteries are charged individually then connected in parallel if one has charged more than the other when connecting together the voltage will balance, this is when large circulating currents can flow, causing damage, charging together if one side starts to charge faster it automatically takes slightly less charge (current) from the charger in favour of the other side which will charge faster until it catches up, under load a similar process takes place ensuring they both stay balanced.

One drawback of using diodes remember a diode looses about half a volt across it when forward biased, and with only 7.2v to start with (from the battery) only 6.7v gets to the motor (less the losses in ESC ).

hope  this helps Tom

 

[Dave MilbournParticipant @davemilbourn48782]

Rick

I think this may explain the diodes.

http://www.action-electronics.co.uk/pdfs/Alexander%20Von%20Humboldt%20-%20Terry%20Small.pdf

Tom may be correct in saying that two parallel packs should be OK if always treated as one, but I’ve seen the results of doing so by less-than-careful owners (although I hate to disagree with Tom as he is one bloke who really does know his electrons!).

PM me for further details if you like.

Dave M

[Rick DevonshireParticipant @rickdevonshire]

Thanks Dave,

Schotty Diodes – I seem to remember the name. I must have seen them on a similar diagram as the one you directed me to. Can these be purchased individually?

Am I correct in thinking that unequally charged batteries can produce quite a nasty conflagration?

Rick.

[Rick DevonshireParticipant @rickdevonshire]

Just noticed -There’s a ‘k’ in schottky. Rick.

[Dave MilbournParticipant @davemilbourn48782]

Rick

I don’t know if you can obtain these from a “normal” retail outlet. We buy them in quantity from one of our main component suppliers, where delivery charges and minimum order values aren’t a problem. You will need one diode in each positive battery lead, and they should be powerful enough to carry the normal operating current. As they can get hot then heat-sinks are recommended too. If you want to obtain a made-up board like the one in the drawing then PM me for details.

Discharge from one battery into another can involve a lot of current, so can cause fire in wiring which isn’t up to the job. 

Dave M

[Colin BishopModerator @colinbishop34627]

If you have a twin screw model then you could dedicate one battery to each motor with no crossover wiring. Yoy would of course have to charge them separately.

 

Colin

[Rick DevonshireParticipant @rickdevonshire]

Thanks for all this information, Tom, Dave and Colin.

Rick.

 

 

[Phil Winks 1Participant @philwinks1]

now I have to say the issue of x over wiring and potential back charging has never occured to me or been mentioned by those “experts” that I know, and have many times used both NiMh and LiPo packs in paralell to extend running/flight times in both boats and aircraft and have to say never had an issue. surely two packs in paralell act like one? you don’t see diodes in a 6s2p or similar pack. I should point out that my bty charging/maintenace regime is pretty strict and care is taken to ensure all my custom NiMh packs use properly matched cells with near identical performance characteristics and I watch my LiPo’s like a hawk and only ever balance charge them. it also occurs to me that Schottky diodes while running hot are wasting power (the heat must come from somewhere) another reason for not liking mechanical speed controllers! the ballast resitors waste so much energy, much better to leave it in the bty when on half throttle than use it to warm the inside of the model. Also while I’m not familiar with the esc featured in the diagram above it appears to have a BEC circuit so why is there a need for an RX supply from the distribution board? maybe its an Amps issue?

[Dave MilbournParticipant @davemilbourn48782]

Phil

I think that the perceived need for your strict charging and maintenance regime anwers most of your own questions. I don’t think that many modellers appreciate the querks of rechargeable batteries and so don’t go to the lengths or expense which you feel are required. Often the cheapest NiMH cells will do – frequently of different capacities and manufacture. Computer-matched packs and balanced LiPo chargers are on a different planet to most of our regulars!

I have been sent ESCs for repair where the copper track was blasted away from the epoxy board by the power of one parallel battery discharging through it into the other, and also seen wiring which had melted in the heat from another similar incident. It happens.

The ESC in the diagram is not fitted with BEC, hence the 5v regulator fitted to the distribution board. Works well and is very popular, if sales are any indication.

I couldn’t agree more with your comments about mechanical speed controllers!

DM

[Phil Winks 1Participant @philwinks1]

Hi Dave thanks for the comments. the reason for my bty care regime has to be the responsibility of my tutor a man of welsh decent and aircraft enthusiasm however on the plus side like you he knows his electrons and only having a schoolboys elementry knowledge of electronics I tend to follow his advice and look after my bty’s especially as replacement can be so costly. Maybe I ought to be looking at where I use parralell packs. to be truthfull though most of them now have a dedicated parralell pack custom built.

 

Futher to the discusion on the esc I gather from this that unlike an esc with a bty eliminator circuit this one needs the red/+ wire to supply it as opposed to the usual practice with bec’s in that if 2 esc’s so equipped are used for twin motor set ups then one of them should have the + wire disconnected suggesting that in the case of a bec the + wire is simply to power the rx????

Edited By Phil Winks 1 on 02/12/2009 20:15:51

[Dave MilbournParticipant @davemilbourn48782]

Phil

 

Confucius he say “If it works then don’t fix it”. Milbourn he say that chosing between two so-called experts’ opinions often reveals the flaws in both of their cases – so you’re worse off than before. Stick with Confucius – he knew his electrons, too!

 

Ref the BEC – yes, the red wire in a BEC-equipped ESC feeds a regulated (5v) supply from the main motor battery to the receiver. In a non-BEC ESC like the Condor, the red wire is the power connection FROM the Rx battery pack to the logic circuitry in the speed controller. If you want to suffer more then have a look at this link

http://www.action-electronics.co.uk/pdfs/BEC.pdf 

 

Dave M

Edited By Dave Milbourn on 03/12/2009 10:20:20

[Phil Winks 1Participant @philwinks1]

thanks for the link Dave The design function and cons of the BEC I am familiar with and in the interests of a glitch free performance I either install a seperate rx pack or use a 5v 3a Ubec and in both cases disable the bec on the esc I also as you suggest in that document I keep the esc to bty lead as short and fat as poss (typically about 60mm max). also ferrite rings on the esc to rx lead. After checking out your site I reckon I’ll be shopping with you in the jan sales if you have a single switch module that will work on a switched chanell

[Dave MilbournParticipant @davemilbourn48782]

Phil

Include the code “P43” in your letter to Santa.

Dave

[Phil Winks 1Participant @philwinks1]

Thanks for that dave. one poss prob I want it to switch a water pump. see my build thread this pump draws, according to my wattmeter, 1.6Amps could I possibly fit a heavier relay to over come this? also does this one come it kit form?

While I’ve got your attention Dave how about a voltage sensing unit that via 2/3 led’s/bulbs shows the state of the bty ie: green amber red in sequence as the voltage progressively drops? actually it looks like I should be disscussing this on the phone

Edited By Phil Winks 1 on 04/12/2009 10:13:27

[Dave MilbournParticipant @davemilbourn48782]

Phil

You’re right. We’re dragging this thread off topic (Admiral Bish will be onto us…..). Unless anyone else out there is madly interested, I’ve sent you a PM so we can take it from there.

DM

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