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Old 6th Nov 2009, 20:55   #1 (permalink)
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Question Battery Charging differences: Lead Acid vs. Ni-Cd

During your aircraft familiarization course differences in battery types are pointed out with their specific properties (charge, thermal runaway etc.). But what about charging the battery? Is there any (technical) difference charging a lead-acid battery versus a Ni-Cd (apart from heat development regarding the Ni-Cd)? Would your car battery charger work on both types? Is it basically [Battery Voltage] * 1.2 = Charging Voltage ?
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Old 6th Nov 2009, 22:10   #2 (permalink)
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I'm not familiar with aircraft batteries but in general..

Lead Acid and NiCad have very different charging requirements. Lead acid batteries are normally charged from a constant voltage source (with current limit). Nicad and NiMH cells are charged at a constant current with charge state monitoring or "voltage peak detection".

You can only use a charger designed for that battery chemistry and capacity. It's particularly dangerous to use the wrong type of charger. You are virtually guaranteed to have a fire with NiCad or NiMH calls and for sure with many lithium cells.

Even if using the right charger for your battery chemistry you must be careful. For example a charger designed to charge one capacity NiCad battery may destroy a much smaller capacity NiCad battery. The charge rate can be important as some cells are designed for fast charging and others not.

Last edited by cwatters; 6th Nov 2009 at 22:21.
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Old 7th Nov 2009, 01:32   #3 (permalink)
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In addition, nickel-cadmium aircraft ships batteries are serviced periodically, using the deep-cycle method.
Very necessary for continued long term service.
Also, normally in service (IE: in the aircraft), they are pulse charged, using a higher than normal DC voltage.
38VDC, in one aircraft type (L1011).
And, these batteries should absolutely not be stored in the same environment with flooded lead-acid types.

Many precautions, with bad results (read: danger) if not followed carefully.

Many pilot dismiss their ships battery (or, batteries), by perhaps thinking...'it's only a battery, no harm, no foul.'

VERY far from the truth.
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Old 7th Nov 2009, 04:27   #4 (permalink)
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Yes, I can remember being very uneasy about some of the first NiCads.

411A BTW, what was the nominal voltage of the units being hit with 38v?
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Old 7th Nov 2009, 04:32   #5 (permalink)
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411A BTW, what was the nominal voltage of the units being hit with 38v?
28VDC, as I recall, 38VDC pulse of six seconds, with time in between pulses...can't recall how much, offhand.
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Old 7th Nov 2009, 05:04   #6 (permalink)
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Nicad batteries like to be discharged completely (deep cycle) to maintain their strength. There are specific machines to do this and they also monitor the temp because a thermal runaway can be a very ugly thing. Just doing a topping charge eventually reduces their output.
Lead acid batteries do not like like to be drawn down hard.
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Old 7th Nov 2009, 06:07   #7 (permalink)
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Hi Junkflyer and others...

The equipment I use for my flying hobby (when I get the time), uses ni-cad batteries.

I 'deep cycle' them around every 3 months...

The discharging/charging equipment I use does this very efficiently.

The point I have here is in reference to your comment,

'..like to be discharged completely (...',

that, in my experience, is never the case.

Ni-cad cells are discharged to 1.1v and then immediately recharged!

Has kept mine going for quite some time!

I simply top them up between each use during the 3 month usage.

I simply work out their percentage efficiency to decide whether to retain or ditch them (recycle of course).

I need absolute reliability!

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Old 7th Nov 2009, 06:59   #8 (permalink)
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Nicad batteries are excellent for situations that require high current delivery for a short time. IE Starting a jet engine, thay also maintain there nominal voltage for a longer time, followed by a sharp drop off.. Lead acid bateries delever a lower current for a longer time, but votage starts to drop off quickly.

Servicing of an A/C L/A battery usualy involves charging at a fixed current for a certain period of time or until the specific gravity of the cells reaches 1260. Usualy the tested for capacity, and recharged.

Nicads can either be charged at a constant current, at a combination of constant current/constatnt voltage or reflex charging.
Reflex charging involves charging at a high current (1 Hr rate) with pulses of discharge. This process helps charge with reduced heat generated, and helps minimise the memory effect..
Constnt Voltage /Current involves setting the charge voltage at about 1.4 volts per cell, and then setting the charge current. The current will be maintained until chg voltage is neared, and the current grops off.
Constant current is exactly that, a current is set (usualy at the C4 rate : ) and continues for 150 % of the battery capacity.

Deep cycling of a nicad involves totaly dicharging until the individual cells are at zero. As the cells reach 0 they are shorted out to prevent charging in reverse as this will destroy the cell. The battery is usualy left for at least 16 Hrs with the shorting strips on to stabilise the cells. Recharging and cap testing follows.

These basically the methods I have used over the last 30+ years on Mil A/C batteries. Procedures have changed a bit with the introduction of newer batteries and chargers.
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Old 7th Nov 2009, 17:09   #9 (permalink)
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Regarding the "discharged completely" comments above...

Lead Acid - Don't do it. Fast way to ruin a lead acid is to deep discharge it and worse to leave it flat for any length of time. Even so called deep discharge batteries will live longer if you don't let them go flat. Batteries that aren't going to be used for awhile (eg lawnmower batteries over winter) can benifit from being stored on a float charger to stop them self discharging. This is a constant voltage charge at slightly lower voltage than that used for full charge. If you use the float charge voltage recommended by the manufacturer you can leave them on float charge all winter - just need to check the water level. If the voltage is right they won't boil/fizz much if at all.

NiCAD/NiMH - Depends what you mean by completely. Deep discharging can sometimes rejuvinate these cells BUT beware.. It's not advisable to discharge a battery (containing many cells) down to 0V because some cells in the battery may get reverse charged. For example you might get to a situation where the cells in a four cell battery end up at 0.1V, 0.3V, -0.1V, -0.3V (total adds up to 0V). Best to stop at 1.1V per cell or monitor the voltage of each cell individually.

Lithium Polymer - Don't do it. Never discharge below about 3V per cell on load. Some types should not be recharged if the voltage has been allowed to get too low.
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