Litium batteries conversion? - Page 1
Forum section: electronics / beginners

author: Fiddle_er
author_rank: Contributor
timestamp: July 25, 2016, 01:39:58 pm
content: i got roped into doing the hedge trimming for my next door neighbours. He had cordless trimmers, i started the job only to find they died after 2 or 3 minutes use (that's the trimmers not my neighbours). Having paid 90 quid for the trimmers and only used once or twice a year and now only 3 years old, would one expect batteries to be duff? Me, being kind hearted said i would sort it out. Though It did not take long for my knowledge to falter. i know they were charged prior to being packed away and stored in house. i know they were recharged before use. I know the voltage is showing 20.2 volts, i opened the pack to find 5 off 3.6 vdc lith iron battery cells, i measured each one all 4.0vdc or alittle over. the electronic monitoring pcb inside did smell a little when sniffed up close but i decided that it was not bad and i did not see any burnt damaged components, well obvious ones anyway.Rant, This has happened to this chap several times and these battery packs really annoy me because the cells could be ok but the control could be duff , but most people dont have any idea and throw away repairable expensive stuff!what can be done? on the cheap, my thoughts; use a 12v car battery and connect direct to trimmers, though it is 18vdc Li iron bat, although probably work, the motor looks so feeble and the current draw would i suspect burn the motor out.Next, thought to get a transformer, but the cost ? well the battery pack looks to be rated 1amp/hr but each cell is 2400mA printed on them. so you are looking at big transformer, big diodes for rectifier, big money?i thought a bout upgrading the cells but then the control pack would be affected.Please, any good ideas or help, how many trimmers does my neighbour have to buy?

author: Assafl
author_rank: Frequent Contributor
timestamp: July 25, 2016, 02:08:43 pm
content: 2400mAh doesn't necessarily relate to a high current.For example, if the trimmer is designed to work for 1 hour, it will 2.4 amps. Not too much. If it is meant to work for 2 hours, the current drop will drop to 1.2A. So not a hefty supply at all.18VDC is 18VDC. That is what you need to supply it with. Be it with 3 6V cells or an 18V power supply.If all cells are charged - what is the voltage on the battery pack? Is it not 16-18v?As for battery powered stuff - only ever get rechargeable stuff for things you use daily or weekly. Most batteries - including chargeable - have limited life span. So if you plan to use something twice a year - either get mains powered or regular batter powered (AAs or similar).As an example, my impact driver and drill are 10v Makita (smallest that will work & share the same battery & used weekly). But the impact hammer/drill is a mains - I use it a few times a year - and I really wanted the 28 or 36V Makita - but it would not survive the charging discharging cycles and would end up being very expensive. If however, you do own stuff that you don't use often - power them up monthly. Use them a little and recharge them.

author: Fiddle_er
author_rank: Contributor
timestamp: July 25, 2016, 03:38:52 pm
content: Quote from: Assafl on July 25, 2016, 02:08:43 pm

author: Kilrah
author_rank: Unknown
timestamp: July 25, 2016, 06:07:47 pm
content: 5 2.4Ah cells in series is still 2.4Ah, it's the voltage that's increased.

author: Fiddle_er
author_rank: Contributor
timestamp: July 25, 2016, 06:56:08 pm
content: Thanks "Shocking" memory

author: Fiddle_er
author_rank: Contributor
timestamp: July 25, 2016, 07:37:28 pm
content: Anybody help me here with a circuit for mains to 18vdc fixed regulator supply ie what bits should i use, it 's for the hedge trimmer that has max current of 2.5 amps and the battery i am replacing was 18vdc

author: Chalcogenide
author_rank: Regular Contributor
timestamp: July 25, 2016, 08:20:38 pm
content: Many laptop power supplies provide 19V at 4.7A (90W), although I would double check the current under load, because 2.5A looks suspiciously low for a trimmer.

author: Fiddle_er
author_rank: Contributor
timestamp: July 26, 2016, 11:22:27 am
content: Quote from: Chalcogenide on July 25, 2016, 08:20:38 pmnice idea , i do have pwr sply for lap top doing little , its 19vdc 3.16 amp output.not sure how to test trimmer under load, maybe trap some wires in battery connection to a multimeter?

author: captainscarlet
author_rank: Contributor
timestamp: July 26, 2016, 12:08:53 pm
content: That's one way.You can tell by the sound. If it sounds like it's struggling then you need more current.

author: MosherIV
author_rank: Super Contributor
timestamp: July 26, 2016, 12:28:16 pm
content: HiWhen an electric (DC) motor (with armature) stalls, the current draw goes up. An easy calculation to do, is to measure the DC resistance of the motor and work out what the current is going be through that resistance.Batteries (so long as they are not flat) can easily supply the extra current.PSUs on the other hand need to be capable of supplying the extra current.I know from testing switching PSU that they can withstand a fair amount of over current BUT they will shutdown if too much current is drawn.

author: Buriedcode
author_rank: Super Contributor
timestamp: July 26, 2016, 01:14:10 pm
content: I very much doubt the trimmer will work for 1 hour continuously, or even a total of one hour used sporadically. My parents have one and it works, can be used for about half an hour of trimming, but because it starts up to quickly and almost instantly stops its designed for very short bursts, being left on for maybe 10-15 seconds at a time. You have to wonder how such a small light battery pack can power something like a hedge trimmer - it just uses a lower power motor, and the manufacturer 'assumes' a user won't hold down the button until it dies. The high current draw will reduce the capacity of the battery, so 2.4Ah will be more like 1Ah, probably drawing more than 3A. The same is true of portable vacuum cleaners, they get away with making it small and light by being quite hard on the batteries, and telling users to use it in 'short bursts' - that's why there is no latching switch as you would have on a mains device, but rather a momentary button. The effective run time is matter of minutes, but is the price one pays for portability.The laptop PSU idea sounds good to me, with 19-19.5V almost a standard. I would have thought the motor, if not running for more than 15-20 seconds at a time won't mind the extra 1-1.5V. But of course this should never be put in parallel with the batteries. As the batteries you mentioned are nominal 3.6V, their peak voltage is 4.1V, 5 x 4.1 = 20.5V. This will drop considerably when under heavy load, probably down to 18-19V, so the laptop PSU I don't think will upset the motor.

author: Fiddle_er
author_rank: Contributor
timestamp: July 26, 2016, 07:22:50 pm
content: thanks, laptop psu it is then, even cheaper than buying transformer some caps and a regulator and of course a box. thouhg i would still of like to of givrn it a go ...

author: Kilrah
author_rank: Unknown
timestamp: July 26, 2016, 09:38:45 pm
content: Again it's not guaranteed it will work.