Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[Bob K]

I am running two 12V NiMh 2600mAh packs in series to run a 24V mister unit.
Ideally I would like to also charge them in series via a single charging socket to avoid having to keep taking them out to charge.
 
Has anyone used a Mascot 8714 'Medical' charger?
In the tech spec it says
      Output Voltage: 24V
      Output Voltage Max: 24VDC
      Output Voltage Min: 12VDC
      For Use With: NiCd or NiMH Batteries

However, when it arrived it had "for 1 to 10 NiCad cells" on the casing, indicating only 12V.
A multimeter across the output connector showed very high DC voltage at zero load.
 
I have tried contacting the supplier to ask, without success.
 
Should I "risk" the battery packs / charger by trying to charge them with this?
 

[Netleyned]

Having read the spec supplied by Farnell I would say Yes as your batteries are 2600mAh 24 Volts and the charger can charge
from 500mAh to 4000mAh
All you have basically are 20 1.2V cells in series.
If your multimeter will read 10 A on current it might be worth putting it in series to see what the charge rate actually is.


Ned

[Bob K]

Thanks Ned.  I will give it a go

[inertia]

Thanks Ned.  I will give it a go

Bob - please read my PM before you do.
DM

[Bob K]

Thank you DM.  PM advice greatly appreciated. 

[malcolmfrary]

Reading the Farnell sheet, at http://www.farnell.com/datasheets/301322.pdf

Constant current charger
for 1-10 NiMH/NiCd celler
• For 500 mAh-4 Ah batteries
(C/10 charging current)
• Linear
•
Thermal fuse in transformer, thermal
protection in regulator, protected against
reversed polarity and short circuit proof
I would assume that the "24v max at no load" that gets a mention further down the sheet in the Tech Spec section is really a warning.  It does say that its a constant current charger, so the extra 12 volts between "minimum" and "max no load" is probably accounted for by the constant current circuit.  I would not be expecting it to fully charge a 24 volt battery, since a fully charged one will be nearer 28 volts.  Also, I didn't see any mention of auto shut off, so presumably it will continue to charge at the set rate until either you remember to turn it of or something goes pop.
I've just been looking at component-shop's site and noticed that they had a selection of switches, one being a 4 pole changeover.  This could be used to swap between series and parallel in situ, while at the same time ensuring that the charger never sees the batteries in series.

[Bob K]

Thanks Malcolm, this does back what I heard elsewhere today.  a "24VDC" charger is not up to charging a 24VDC battery.
 
I did consider a 4 pole C/O switch, but without make before break contacts I cannot guarantee I will not get a momentary short whilst switching over.

[malcolmfrary]

Make before break would ensure a short, thats what they were intended for.  4 poles of regular change over, however, does the job. A few tags are redundant.

[malcolmfrary]

A quick sketch using the 4PCO switch.  Contacts shown set parallel for charging, all contacts move together, so the rule is that all breaks break before any make makes, no shorts at any time, but obviously a brief break during change-over.

[Bob K]

Sorry, I did mean break before make.  Adding in the two 12VDC charging sockets would use all 12 switch tags if everything is wired to the switch.  Getting a bit complicated.
 
Incidently, I just had a call back from the charger manufacturer.  He comfirmed that to charge twenty 1.2V 2600 mAh cells in series would require 40VDC output at 400 mAh.  So charging in series is out.
 
I may have to revert to using a single larger 12VDC battery plus a 12 to 24 V converter.

[grendel]

how much current do you need at the 24v output, could you not get a cheap in car laptop charger, these convert up from 12v to 20+ V I got one from maplins for £5 (on offer) that does 120W
Alas no longer on offer
Grendel

[malcolmfrary]

Adding in the two 12VDC charging sockets would use all 12 switch tags if everything is wired to the switch.  Getting a bit complicated.

Why 2 12v sockets?  The switch drawing connects the two batteries in parallel for charging as one unit, then connects them in series to offer them to the load a a single 24 volt supply.  Used like that, provided that they are at a similar state of life, they will always be in the same state of charge, so the problems of connecting batteries in a dissimilar state vanish.

cheap in car laptop charger, these convert up from 12v to 20+ V

20+ volts doesn't quite do it trying to charge a 24 volt battery.  It would need at least 30+ volts.  Some of the Lexmark printers (if memory is working right) had power units that gave this and could be tweaked to do the job without any messing wiring a switch up.

[Netleyned]

I think Grendel was advocating the in car unit to be fitted in the model running off one 12v battery
to drive the mister.MMB can supply a unit to run a 24v mister from a 12v supply

Ned

[Bob K]

Why 2  12v sockets? 

That was so I could charge each and check their status independantly via the sockets.  A fair bit of disassembly to get them out.  Otherwise charging in parallel would work as you say.