Mailing List Archive

Mailing List: techdiver

Message Display

Hi,

I'm reading you loud and clear but it seems your local mailer thinks
I've changed my e-mail address: I'm unable to mail you directly.

I sent you two huge files from a different account;
A NiCad info file and the UK light conversion/mounting instructions.
I'm unable to tell from this end if you got it.
Please let me know if my alternative path made it through to you.

If there is a problem I'll include both files in my USPS package.

You might want to direct the battery questions to a bigger audience.
Also, Radio Shack sells a book called "Power Supplies" which should
cover some of the issues you are facing when building a charger.

Just to keep you going:
>Charger circuit: includes a 15 minute timer in series with three
>ceramic resisters.  Each resister has the following written on them:
>   20 W
> 0.37 Ohm J
> R  I  O

I read this a 0.37 Ohms resistor rated at 20 Watts.  ( Only high
quality ohm meters can measure such low resistance).
I assume the resistors are in series - that makes them effectively into
an 1 Ohm (3x0.37),  60 Watts (20x3) Watts resistor.
>The charger specs are 1200mAhours (7.2 V).
I think the writing on the charger tells you what batteries to charge:

Firing this beast from a car battery:
4 volts difference (12.7 - about 8 on the pack) over a 1 ohms resistor
means the current is 4V/1 Ohms or 4 Amps which should fill a 1200mAh
cell in about 20 minutes. 15 minutes gives you a bit of safety margin,
especially if the cells have been abused and have lost capacity.

If you hook 8 4Ah D cells up to this charger you get about 1.6
Volts over the resistor which pins the current down to 1.6 V/1 ohms
or 1.6 Amps.  Your 4Ah cells needs 4 hours of charging at this rate
*if* they were empty at the beginning of the charge cycle.
It can be hard to determine if the cells are empty but the limiter
when installed has a feature which will tell you if your batteries
are low and in need of charge every time you turn the light on.

To get three hours of fast charging out of the charger you are
considering using, you must either redo the timer (?)
or put in a switch to bypass the timer.
NiCads rated for charging at 400mA will suffer internal damage
and permanent loss of charge storage capacity if *overcharged*
at a rate higher than 400mA. So, your cells need to come off the
high charger current _before_ they are full.  I find that
by adding about 5 ohms in series with the existing 1 ohmer and
assuming that all 8 cells are being charged in series,
that the current is below 400mA (C/10) and is thus at a safe
level for both charging and overcharging.

So, assuming the cells are about empty and you need them fast:
go for 1.6 Amps for say 3.5 hours
and then switch in the a 5 ohms/10 Watter top them off for
a few hours unless the next dive is calling.

If you don't need the cells charged fast, let them charge at the lower
current setting.

Observe that 6.5Ah can be a significant fraction of the capacity
of the battery from which you charge the NiCads.

john

Navigate by Author: [Previous] [Next] [Author Search Index]
Navigate by Subject: [Previous] [Next] [Subject Search Index]

[Send Reply] [Send Message with New Topic]

[Search Selection] [Mailing List Home] [Home]