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For a fail-safe 24 V dc auxilary supply, is it better to use 12 batteries of 2 V each, or two 12 V batteries?
March 9, 2007
First, let’s get your terminology correct. Most people, even engineers, call all those little power packs “batteries,” from the little things powering your wristwatch to the big monsters that pushed World War II submarines. In fact, the little-bitty things in watches and calculators are “cells.” All of the AAAA, AAA, AA, D and C “batteries” are also “cells.” The 12-V brick providing startup power in your car is a “battery,” however. What’s the difference?
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| Electrically, there is no difference between twelve 2 V cells and two 12 V batteries. |
The word “battery” has the general meaning any large group or series of related things according to the most general definition (per Dictionary.com), which predates modern battery technology. For our purposes, a “battery” is a set of identical cells connected in series to sum their voltages. They use identical cells because the current produced is limited by the plate area in the smallest cell.
Therefore, one thing is certain: your 24 V auxiliary supply is a battery of 12 lead-acid cells producing 2 V each. Everything else is packaging. You can hunt down a bunch of individual 2 V cells and connect them in series, or you can buy two 12 V batteries and connect them in series, or you can buy one 24 V battery. Electrically they're all the same.
Two-volt lead acid cells are pretty unusual. Offhand, I cannot think of any common use for the things, but they do seem to be manufactured. An Internet search on “2 V battery” turned up exactly one source (out of 36 million references, most of which keyed on the word “battery”). BatteryWholesale.com lists 6 part numbers for 2 V cells, the least expensive being a Hawker D CELL, 2 V, 2.5Ahr “Pure Lead Cell” p/n 035-0073 priced at $9.75. Adding a quarter for tax, shipping, etc., gives $10 per cell, or $120 for your 24 V battery.
Compare that to $84.99 for a Duralast Gold 8AMU1R 12 V battery with an 8-year warranty to fit my 2001 Mazda Miata. Rounding that up to $90 for incidentals puts the 24 V auxiliary supply made up of two 12 V batteries at $180—a saving of $60 or 25% of the individual-cell approach.
The only comparable 24 V battery I could find was an Optima Batteries TF1800 available from Apex Battery for $410.95. It’s designed to power electric trolling motors for small fishing boats. The Apex Battery Website points out that the unit “combines two D34M marine batteries in a rugged ABS polymer outer case….” In other words, it’s two 12 V batteries, anyway!
To sum up, there’s no technical difference between using two 12 V batteries instead of twelve 2 V cells, but there’s a real economic difference. Since battery makers pump 12 V batteries out in vast numbers to satisfy the automotive market, production efficiencies make them significantly less expensive than any comparable solution.
C.G. Masi, Control Engineering senior editor, charlie.masi@reedbusiness.com
For more information about auxiliary power technology, visit the Control Engineering Website at http://www.controleng.com.
Search batteries at http://www.cesuppliersearch.com.
For additional information, visit these Websites:
http://dictionary.reference.com
http://www.apexbattery.com
Posted by Charlie Masi on March 9, 2007 | Comments (3)
In response to: For a fail-safe 24 V dc auxilary supply, is it better to use 12 batteries of 2 V each, or two 12 V batteries?
Andrey commented:
Thank you for excellent comparison. In your opinion are both batteries reliability and availability results comparable with cost results? RESPONSE: If you buy my argument that consumer sources generally provide the lowest price and highest value (reliability/price), then you should consult Consumer Reports to choose among automotive batteries. Otherwise, I recommend polling the top 3-5 name brands, select potential models based first on how well their electrical specifications (amp-hour rating, maximum sustainable current and maximum peak current) match your system specifications, then look at the warranties. --- CGM
In response to: For a fail-safe 24 V dc auxilary supply, is it better to use 12 batteries of 2 V each, or two 12 V batteries?
Paul Jusak commented:
I don''t get the math: Individual cell cost at $10/cell times 12 cells = $120 per 24v battery made up of individually purchased 2v cells. Individual battery cost of $90/battery times 2 batteries = $180 per 24v battery made up of 2 purchased 12v batteries. The car batteries made in the millions calculate to be $60 more expensive than the batteries made with individual cells. The math isn''t right, but your thinking is, since batteries are sized by amp-hrs. What is the amp-hr rating of the 2 car batteries versus the 2.5 amp-hr rating of the individually purchased cells? To make an equivalent capacity battery with the individual cell method should be far more expensive than the car battery method. You were probably thinking, 24v means 24 cells, so 24 cells times $10 = $240 for the battery of individual cells, which is $60 more expensive than the car battery route?? Unless I''m really missing something here? Blame it on BCD math done on a PLC using an octal base. RESPONSE: DANG! That's what I get for writing these things fast! And, my editing partner didn't catch it, either. What I did wrong was do the math backwards ($180 - $120 = $60) You're right that the difference is the amp-hr rating. --- CGM
In response to: For a fail-safe 24 V dc auxilary supply, is it better to use 12 batteries of 2 V each, or two 12 V batteries?
J. Schott, PE commented:
The discussion of (nominal) 2V. lead acid individual cells reminded me of an advertisement I have seen in a magazine devoted to yachts and large boats. These have large batteries in banks to produce 12 or 24 VDC power for the vessel when at anchor, etc. A new product advertised is a system of large individual 2V. cells in a special containment box to replace the usual 6 or 12V. house batteries. The advantage touted is that the individual cells are easier to install and service. This may be a valid point since boat batteries are usually located in some niche below decks where a 50 lb. battery would be difficult to move. Another consideration (from the boating world again) is the difference between deep draw batteries and starting batteries. The usual automotive battery is built to provide a very large amp draw for a short time. It is not built to withstand being nearly completely discharged before recharging. Many boats use a separate battery for starting and a deep draw battery for house power. A golf cart battery is an example of a deep draw battery. For a back-up power supply a deep draw, rather than a car battery, would usually be the better choice.