battery charge.

[colin seniorParticipant @colinsenior17468]

[colin seniorParticipant @colinsenior17468]

Bought a battery powered boat 4 months ago. Try to use it for the first time last weekend.

Encountered a problem.

Found a charger with the correct fitting to the battery connection. Left it on for 24 hrs. – inadvertantly.

Batteries were very , very hot – expanded the plastic shrin k wrap around them..

 

Maybe its the wrong charger, as nothing now works.

 

Battery pack shows  Ni-Cd.  OPT AA800 mah  9.6 V.

 

Charger reads.

 

AC/DC ADAPTOR.

 

MODEL.  4116-0740-4DC

 

V.INP-230- 50 HZ

 

V.OUT – 7.5V – 400mA.   P.OUT 3VA.     CAN ANYONE HELP- THANKS.

[ashley needhamParticipant @ashleyneedham69188]

Colin. OOPs!!   you may have overdone the batteries a bit! They should not get quite that hot and may now be frazzled past their best (dead).

 

The other thing is that in any case the charger is not quite right as the output voltage would have to be a bit higher than that of the batteries they are charging.

 

I think realistically you need some new batteries and a dedicated charger. If you were going to use the boat for anything more than once in a blue moon, a 9.6v pack of NiMh batteries would not be expensive, and could have a much greater capacity. Batteries fitted to rtr items inevitably have the cheapest low capacity batteries fitted to keep the cost down.

 

I have an MLRS tank r/c toy with a 9.6v pack, NiMh I think, but only 700mAhr,which lasts for about 15 mins if you are lucky….I could get a straight replacement pack of three times the capacity and enjoy a decent play…but I hardly use it so the cost is unwarranted.

 

the only thing you would have to watch is the motor getting too hot on the extended run times you may be getting.

 

Ashley

[Paul FreshneyParticipant @paulfreshney24971]

Colin

Ashley is perfectly correct – it would seem that inadvertently you may have endeavoured to charge 800mAh  capacity cells at 400mAh or so for 24 hours. Therefore, melt down has occurred. As Ashley says, realistically you will need  need new batteries and an appropriate charger.

May I suggest you contact either ModelPower or Component Shop (they both have very good internet sites if you just Google the name) to see what they can offer, or  if still unsure once you have viewed their products, you could email them for product advice. Alternatively, try a good local model shop or Maplins perhaps if there is one near to you?

Paul Freshney

Editor. 

[ashley needhamParticipant @ashleyneedham69188]

Colin, just been Googling Maplins, and their 9.6v batt  pack, which although NiMh (and so a bit better that the old NiCd packs) appears only to be 800mAh and so no better than your  old pack. Their charger is a fast charger as well, which is never a very good idea unless you are racing and are desperate for recharging on the quick, as it shortens battery life.

 

If ordering on line, you will need to specify the layout of your pack and also the connector on the end. The Tamiya one is the common sort. From what i remember, the Component shop has pictures of all the bits and layouts so you should not have a problem in this regard.

 

Ashley

 

 

[Mark Beard 1Participant @markbeard1]

One problem with common-or-garden mains adapters is an issue called regulation.  In short, (no pun intended), this means that they are designed to give their rated voltage when supplying the rated current.  At less load current the output voltage is higher than rated voltage.  I think this can explain why a ‘7.5V’ adapter was able to shove current into a 9.6V battery.

 

By far the safest way to charge any NiCd or NiMH battery is to trickle charge for 14 hours, from ‘fully’ discharged.  By fully discharged I do NOT mean flat!  I mean run down to 1.0 volts per cell (or 83% of rated voltage).  Any lower and you risk damaging the pack.  (Not all cells are equal, in deep discharge any cells with more charge may force current the wrong way into weaker cells and cause them to reverse polarity => dead pack).

 

 You can make a trickle charger easily from a mains adapter which has a higher voltage than your battery pack.  You need to charge at a current which is 10% of the Ah rating of the pack, (to give a 10 hour charge rate – charge over 14 hours, the extra 40% makes up for less than 100% charging efficiency).

 

To control the current you need a resistor.  Measure the off-load voltage of your adapter (as noted above, don’t rely on what’s printed on it since most likely you won’t be using it’s full rated current).  Then subtract the battery voltage from the adapter voltage to find how many volts to lose across the resistor.

 

To calculate the resistor value, divide the resistor voltage by the charging current in Amps (10% of the Ah rating).  To check that the resistor power capacity is sufficient, the resistor power must be more than the resistor voltage x charging current.

 

For example: I want to charge a 9.6V 800mAh pack from a 12V adapter, which measures 13.2V off load.  The charging current is 10% of 800mA which is 80mA or 0.08A.  The resistor must drop (13.2 – 9.6) 3.6V.  Dividing volts by amps gives a resistance of ( 3.6 / 0.08) 45 ohms.  Resistor power is (3.6 x 0.08) 0.288W.  The nearest standard resistor value to 45 ohms is 47 ohms.  A standard 1/4W (0.250W) resistor will get too hot at 0.288W so I need a 1/2W 47 ohm resistor.  QED.

[Mike DavidsonParticipant @mikedavidson22772]

Colin, it seems to me that something occurred to  make too much current flow through the battery cells. If the cells were hot enoughto stretch the shrinkwrapcovers,the same amount of energy must have been flowing around the charger. It is possible that there was a thermal fuse was fitted to the charger. In my mind, I feel there should have been a resistor to limit the amount of current drawn from the devicebut as has been said by a previous correspondent, there seems to be a lamentable lack of regulation with the plastic encapsulated devices that have a set of plug pins to fit a 13 amp socket  .  I would suggest opening the unit to search for damage or failure, and unless you find an open circuit thermal fuse, be prepared to throw the unit in a suitable binand see if you can buy a purpose built charging unitthat is designed to charge 9.6 volts, and, READ THE INSTRUCTIONS…..   Mike Davidson in Plymouth

[Dave MilbournParticipant @davemilbourn48782]

Did I read that you’re charging a 9.6v pack with a charger whose output voltage is 7.5v?   If so then I’m not entirely surprised that nothing now works.

Like Mike says, bin the lot and buy mutually compatible items from a reputable supplier.  Then read the instructions.

If, like some of us, you make a habit of inadvertently forgetting to switch things off  then use a plug-top 24hr timer to switch the charger supply off after the requisite time.

Works for me!

Dave M

[ashley needhamParticipant @ashleyneedham69188]

David has a good point there with the timer trick.

 

I never have anything on charge for too long without rigorous supervision and in any event dislike leaving things like chargers unattended in the first place.

 

i should think Maplins would do a suitable timer…..

 

Ashley

[Dave MilbournParticipant @davemilbourn48782]

For those who haven’t yet discovered it, this is a very useful website for timers and all sorts of other electrical and domestic gubbins:

http://cpc.farnell.com/ 

You don’t need to set up a trade account. – just order and pay on-line with the old plastic fantastic. There’s a delivery charge if your order is less than £45 and the prices shown are VAT-exclusive, but I often find stuff at the most unbelievable prices e.g. Low voltage halogen spot bulbs for 70p each (£5.98 for two in Homebase) and USB 2.0 cables for less than £1.

The only drawback is that once you’ve ordered from them they mail you a “Special Offers” catalogue just about every week!

See what you think.

Dave M

 

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