"low-batt" signal

[K-T]

I am constructing this battery pack with the "dimensions" of 19,2V/3,4Ah (2x16cells). I have read somewhere that a recharg. cell should has to be recharged when voltage has dropped to 0,9V. With 16 cells the battery pack would be regarded as empty when hitting the 14,4V mark. At this point the regulator should switch off to protect the batteries.
What I want is the controller to dim at that point of time when only 10-15 minutes of battery power are left. How do I calculate where to set that mark. The bulb is a 12V/35W MR16 pushed to 14,5V.

I hope to have made my problem clear even though it is explained a little bit confusing.

:clueless:

Thanks in advance, Klaus.

 

[K-T]

If I understood you right I will only - if I am lucky and careful with recharging - get about 17V out of the cells (1,07V per cell)?

Klaus.

 

[lemlux]

Klaus:

You correctly understand my assertion. Larger rechargeable batteries will typically have less voltage drop. I have just ordered 6 @ 4500 mAh NiMH C cells and am very curious to measure their voltage drop.

 

[lemlux]

Klaus:

I too am thinking of using a 35W 12 V lamp in a project. I am lazily (and expensively) considering of bolting a PIAA 002X 20 degree beam 35W running lamp and a handle on top of a watertight Otter 3500 case with 8 @ 2450 mAh 6A battery packs wired as (edit) 4 * 12 cells (or 6@ packs in an Otter 9000 pack). At about a 1.0 A draw per cell I would expect to pull about 1.16 (edit) V per cell or close to 13.9 V. I am also considering attaching one of the Carley aluminum reflectors I've ordered and overdriving a 6.0 V 3.35 A bulb in a smaller box with three or four serial 7.2 V battery packs. (I was able to buy a whole bunch of surplus 2100 mAh and 2450 mAh battery packs at between $2.50 and $5.00 each a few months ago.)

Maybe it would be more practical to use a 8.0 Ah SLA to drive a high amperage 9.6 V bulb voltage regulated to a max of 10.5 V. Maybe this is the light for which I should buy a voltage regulator. Decisions, decisions, decisions.

The W/A 01185 9.6 V bulb overdriven to 10.56 V is rated at 3.32 A, 35 W, 1140 Lumens and 16 hours life. When the pulse regulator drops below 10.56 V the light would gradually dim to 817 Lumen as voltage drops to 9.6 V.

The downside is that my 8Ah 12 SLA battery weighs 5 3/4 pounds. 6 pounds would equal 12 @ 7.2 V battery packs or 14.7 Ah of 14.4 V power. 2 * 3 7.2 V packs driving a voltage regulated 9.6 V bulb becomes more attractive as I think about it.

 

[lemlux]

Klaus:

I don't know the answer to your question, but I offer the following comments.

The 0.9 V recharge point is good for one cell. The problem with cell packs is that the individual cells have variable rates of self discharge. I have read that slower charge rates allow the cells in parallel to equalize. I presume, but don't know that a 50 minute trickle charge of 100 mA at the end may encourage the serial cells to equalize as the less than full cells take charge while the full cells generate some heat.

This long preamble is designed as a lead-in to advice that advises you to recharge considerably before you get to 0.9 V average per-cell output under 150 mAh load. You're attempting to pull about 1.5 A. per cell off of cells with a 1.7 Ah capacity. My experience suggests that you'll only get about 1.07 V per cell with freshly charged batteries

 

[K-T]

See, told 'ya. Now there are two people on this board who would like to build one of these LVR-PWM regulators.
Because of the impressive feedback (it does indeed seem to be pretty hard to build one of these units

) on both issues I am seriously considering buying one of these regulators from Willie - then I could spend more time desinging the housing.

Klaus.

 

[lemlux]

K-T:

I've been on the verge of buying a couple of Willie's LVR3I's for a while, but $50 a pop is painful. Oh, well.

The housing remains an issue: The well designed Otter cases weigh 1 pound + and add about an inch in each direction -- I don't need a waterproof container, and I'd like to avoid the bulk. At least one project will be made out of the case and bezel from my 30 year old Daycor 800 which will hold 6 of my 7.2 V packs.

 

[K-T]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by lemlux:
K-T:

I've been on the verge of buying a couple of Willie's LVR3I's for a while, but $50 a pop is painful. Oh, well.

<HR></BLOCKQUOTE>

I fully agree with you. I would have to pay the 50$ plus 5$ shipping to Europe plus customs 20% would make impressive 65$. Maybe we could work out some group buy

Klaus.

 

[lemlux]

I suspect that a group buy would only generate attractive pricing if we all asked Willie to program the devices to the same input and output parameters.

Maybe Willie's 24 / 12 configuration designed to put out between 12 and 14 V from an input of two serial 12 V SLA batteries is flexible enough to take three of my packs (18 cells) in serial as well as two of your packs (16 cells) given the stated 2.5 : 1 range of input to output that works. This might be the most attractive "group buy" for Willie to entertain. If a common output setting affects price, I'd vote for 14V.

 

[jmm]

K-T: I was worried about similar things when I bought my first 12V rechargable hand drill many years ago. I hacked together something from sevearl of the "electronic circuits cookbooks" to do the job you propose, but soon gave up on the idea of making it "interactive" as it would cycle when the load was chopped, and the batteries recovered (I tried time delay/hysterisis circuits, but never got it right). I ended up just having it turn on a red LED when the voltage dropped to 10 volts (1.0 volts per cell). It still works and I'm still using the original 6+ year old Ni-Cad battery packs with that drill. I've had to replace battery packs on newer drills which I have been too lazy/busy to equip with a similar circuit, but I suppose it's hard to say whether knowing when to recharge is responsible, or the battety packs for that drill were just "better".

When I was building it and researching related things, it seemed like most of the individual Ni-Cads in the packs I was using came down in voltage fairly evenly under load, but tended to get pretty far out of whack with each other if they self discharged over a long period of time without being used. I was able to observe that subjecting these self depleted battery packs to any kind of high load without putting an equalizing charge on them first definitely brought on the dreaded "individual cell reverse charge" condition we're trying to avoid by recharging before the pack voltage gets too low.

One additional danger of packs with 10 or more cells is that you could theoretically have one cell at no (or very low voltage), yet still have the pack read higher than whatever low voltage cutoff you eventually decide on under light load conditions (going with .9 rather than 1.0 or 1.1V makes this condition more likely to happen). I think we're fortunate here in that we're dealing with lights, which have more constant or predictable loads than things like power tools. This stuff gets really complicated, not just electronically, but logisticaly.

Later Edit: ----------------

Just remembered that Dorcy now makes some incandescent light models with a battery voltage monitoring circuit built in. They use different colored discrete LEDs to give you a visual indication of battery voltage when the light is on. One model is sold over here under the Sears/Craftsman brand name.

John