Tuesday, February 17, 2009

LEAD ACID BATTERIES: The 411 for Forensic Engineers

Batteries are energy storage devices, so they can be a source for fires or electrical arcing or even shocks and thus are part of what a forensic engineer might be looking at. With hybrid and electric cars some very high energy applications are going to become more prevalent.

Warning: batteries contain chemicals of varying degrees of acidity, alkalinity, or toxicity. Use protective garments, gloves and eye protection when investigating them and be prepared to take samples with containers capable of holding such chemicals without reactions.

All Batteries were originally broken into two kinds: Primary Batteries and Secondary Batteries. Primary batteries are those that have non-reversible chemical reactions, ie, they are not chargeable: Alkaline (zinc-manganese dioxide or oxy-nickel hydroxide), Zinc-carbon. Secondary batteries are those that have a reversible chemical reaction: lead acid, Nickel-Cadmium (NiCd, NiCad™), Lithium Ion, Nickel-Metal-Hydride etc.

All of these have their various characteristics that are of interest for use: cost, (milli-) Amp-Hours, Voltage/cell, weight, size, energy density, power density, etc. For purposes of forensics it is more likely you will be interested in how they work and roughly how they are constructed.

This article will take up the lead-acid battery.

Lead-acid batteries are based upon acid (sulfuric) reacting with metal (lead). There is evidence that metal/acid batteries were known around 2000 years ago. The “Baghdad Battery” or “Parthian Battery” can be researched for those having an interest in the history of batteries. One can also make a battery with a nickel, penny and a lemon or potato. I found it to be about .5 volts with a lemon.

Cells and batteries and packs.

Cells are compartments (just like a jail cell is a compartment) wherein electrical potential can be developed electrochemically. Their voltage can range from roughly 1.2 volts to 2.6 volts. Originally cells did not have enough voltage to be useful so they were combined together in parallel: batteries. E.g. a car battery has 6 cells. The word “battery” comes from using cannons in groups called ‘batteries’ (but the word itself means something beaten or forged, thus a cannon is a forged thing). In more modern uses there are single cell batteries: the common flashlight battery is only a single cell.

A battery pack is when batteries or cells are joined in parallel or in series.

Lead-Acid batteries fall functionally into two major categories: SLI (starting, lighting and ignition) and Deep cycle batteries. The difference being, the SLI type batteries have more plates in the cells to provide surface area for great current generation, but are not tolerant of being heavily discharged. The Deep cycle batteries are optimized with fewer and thicker plates and a greater tolerance for being discharged to lower levels.

Lead acid batteries fall into approximately three versions: liquid (traditional), gel, and AGM (Absorbed Glass Matt).

Liquid (or “wet”) are either maintenance free or serviceable. Maintenance free do not have the handy caps you can loosen and check the chemistry on. These tend to be used for SLI applications.

AGM are sometimes call “sealed regulated valve”, “dry cell”, “non-spillable” or “sealed” lead acid batteries. They are considered to be good for deep-cycle applications, but are also preferred where the possibility of a spill exists.

Gel are so called because there are additives to make the sulfuric acid solution ‘stiff’. They are usually best used for very deep cycle application, but must be charged with a lower charging voltage.

Both the Gel and AGM are also called “Valve Regulated Lead Acid” batteries. These batteries are mistakenly called “sealed” batteries, but that would be explosively dangerous when being overcharged. They are, however, ‘recombinant’ batteries because they are designed to have the generated oxygen (at the positive plates) and hydrogen (at the negative plates) recombine into water, rather then escape, except under sufficient pressure. Thus the ‘regulated’ valve.

Construction: Lead acid batteries are lead plates in a 33% sulfuric acid solution. The plates are alternately connected together within the cells, and the cells are tied together (electrically) in series. (See Figure 1)

Battery Specifications:

Amp-Hours: The amps times the hours it can deliver it. For a 200 Amp-Hour (AH) battery should deliver 20 Amps for 10 hours. In actual practice the slower it is discharged the more energy one can expect.

CCA: Cold cranking amps: Amperage at 0 degrees F delivered for 30 seconds and the voltage maintains above 7.2 volts.

CA: Cranking amps: amperage measured at 32 degrees F.

MCA: Marine cranking amps: Same as CA.

HCA: Hot cranking amps: amperage measured at 80 degrees F.

RC: Reserve Capacity: The number of minutes a fully charged battery can discharge 25 amps, at 80 degrees F until the battery voltage drops below 10.5 volts (for a 12V battery).

Lead Acid Battery Chargers

The best battery charges have three steps in charging a battery:
1. Bulk charging: about 80% of the is charged at the maximum voltage and current rating for the charger.
2. Absorption charge is charging at 14.4 volts until 98% of the total charge capacity is reached.
3. Float Step with a regulated 13.4 volts at 1 amp or less of current. These will charge the battery to 100% and keep it there without dangerous ‘boiling’.

Note: a charger mis-charging the battery can cause damage to the battery.

The VRLA batteries sometimes require special charge curves.

This should give an Electrical Forensic Engineer enough data to investigate a loss involving a lead-acid battery. There is no attempt here to identify all the ways a battery can fail and cause damage. That is up to the Forensic Engineer in the field to figure out. This will, however, give one the terms and some idea of what he is heading in for.

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Monday, October 29, 2007

Better Photography: Fire Scene Documentation

I was on a fire scene last week on Thursday and as usual it was full of burnt objects and debris. It was hard to photograph something and make sure that what I wanted to have stand out in the photograph was in fact standing out. I would look at the image on the back of my camera and...debris! Just debris. So, I would get the fire investigator I was working with to hold it or some such. But I knew there is a another way to make it happen.

A handy way to make something stand out for fire investigators, insurance adjustors and forensic engineers, is to use "depth of focus". This is the fact that a lens can only focus at one distance at a time--which means what it is not focusing on is out of focus.

Okay, sounds obvious, but here's the point: if you use a small aperature (small opening) for the shutter, it focuses deeper--ie, things at different distances will be in focus. If you use a wider aperature (wider opening) you get more of only that thing you are focusing on in focus. This makes it stand out very markedly.

In a confusing fire scene, where you want to draw attention to the thing you are interested in, set your camera to "aperature priority" (usually it is marked "A" on your dial or menu) and make the f-stop as small a number as you can (smaller number, wider opening). Then find an angle on the object that puts everything you are not interested in at a different distance than the thing you are interested in--then take the shot. It will stand out much better!

Here's an exmaple of deep focus with a small aperature in a messy scene of a very large standed cable that experienced shorting:
Small aperature/deep focus

And here's one of wider aperature (lower number f-stop) and shallow focus:
Large aperature/shallow focus

You can see how much better it stands out with the shallow focus.

The idea is to play with this in your office or home until you get the hang of it and just try it at the fire scene when you want something to stand out.

Derek Geer
Forensic Engineer / Expert Witness
San Diego, California

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Saturday, October 20, 2007

Lighting Fire Scenes for Digital Cameras

Yesterday I was in Beverly Hills doing a fire scene with an experienced fire investigator. It was a house where a fire started below the ground floor. The fireman tore up the floor in various rooms and removed sheetrock as usual and dumped it outside.

The scene was a confusion of possible causes. The house had workman in it doing staining of the kitchen cabinets, there was a mass of electrical wires at the area of origin, and after seeing the fire department photos BEFORE it was overhauled we had some very nice inverted-V pattern suggesting liquid flammables.

In any case, it was the usual mess of trying to take pictures in a deep space with a flash. A flash's light falls over with the square of the distance from the flash. This means that doubling the distance decreased the light power to 1/4. Dark deep spaces look like caves in the images.

The thing to do is to turn off the flash. Preferrably lean the camera, brace it, against some wall or door frame or,k even better, use a tripod. The exposure takes longer to happen, but the results are considerably better.

I intend to do a study on this for fire scenes to give some quick rules of thumb to make it easier for insurance adjustors, fire investigators, expert witnesses and forensic engineers to get some good images. Also the trade offs in various lighting techniques.

Derek Geer
Forensic Engineer
San Diego, California

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