+--------------- | PV wrote: | > > However, this brings up a good question: are car batteries suitable | > > for use in
_any_ backup service? Despite their shortcomings, their | > > price/performance ratio might justify the compromises. | >
| > Not really, no. They are designed for a quick jolt of huge current and | > then lots of quiet time. It's like using a sprinter for a marathon. | >
| > For recovery purposes, you use deep cycle batteries. * .... | I understand what you've written, but that's not the question I need | an answer to. Given the price/performance ratio of a car battery, are | there _ANY_ backup applications it's suited for? Think about it: I can | buy them on any streetcorner in the world, they have recharging | stations as close as the nearest working automobile, and they're as | rugged as any battery gets. | | What's the amp-hour capacity of a typical car battery? May I assume | that I can draw _that_ capacitiy for _that_ many hours from a new auto | battery? Can I draw 1/10 that capacity for 10 times more hours?
+---------------
Bill, battery management is a complex issue. For an in-depth answer to your question, I refer you to the lead-acid sections of the excellent resource, especially the sections mentioned below. But the main disadvantage of "typical car batteries" (as opposed to SLA, VRLA, or gel-cell lead acid) is the risk of explosion when used indoors due to hydrogen venting when charging [especially when re-charging from a deep discharge]. Yes, proper ventilation can protect against this, but still.
Anyway, give the following stuff a read. If you really want to use lead-acid, it looks like golf-cart batteries [which, as noted by the previous posters, are built differently from car-starting batteries] would probably be your best cost/capacity tradeoff.
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Can the lead-acid battery compete in modern times?
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Modern Lead Battery Systems
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Charging the lead-acid battery ... When configuring a battery as a buffer, make certain that the battery has the opportunity to fully charge between loads. ... Deep discharges should be avoided if possible. Assure that the float charge voltage is set correctly.
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The secrets of battery runtime ... The internal resistance of Lead-acid batteries is very low. The battery responds well to short current bursts but has difficulty providing a high, sustained load. Over time, the internal resistance increases through sulfation and grid corrosion. ... One of the best batteries in terms of self-discharge is Lead-acid; it only self-discharges 5% per month. Unfortunately, this chemistry has the lowest energy density and is ill suited for portable applications. ...
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Non-Correctable Battery Problems ... A lead-acid battery self-discharges at only 5% per month or 50% per year. Repeated deep cycling increases self-discharge. ... Permeation, or loss of electrolyte in valve regulated lead-acid batteries (VRLA) is a recurring problem. Overcharging and operating at high temperatures are the causes. Replenishing lost liquid by adding water offers limited success. Although some capacity may be regained, the performance becomes unreliable. ...
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How to restore and prolong lead-acid batteries ... Simple Guidelines ...[short list of recommendations]...
Note: Wheelchair batteries don't last as long as golf cart batteries because of sulfation. The theory goes that a golf cart battery gets a full 14 hours charge whereas a wheelchair only gets 7 hours while the user sleeps.
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What's the best battery for wheeled and stationary applications? ... Regular car batteries are sometimes used for cost reasons. There is, however, a danger of spillage if overturned. Neither are regular car batteries designed for deep cycling. ... What's the best battery for stationary applications? Until the mid 1970s, most stationary batteries were flooded lead-acid. The Valve Regulated Lead Acid (VRLA) allowed batteries to be installed in smaller confinements because the cells could be stacked and mounted in any position. Although VRLA are less durable than flooded lead-acid, simple mounting and lower cost make them the preferred battery system for small and medium sized installations. Most UPS systems repeater stations for cell phones use VRLA. Large installations, such as internet hubs, hospitals, banks and airports still use the flooded lead-acid. ...
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Testing deep cycle lead acid batteries ... Lead acid batteries come in two basic architectures: deep cycle and starter types. The deep cycle battery is designed for maximum capacity and high cycle count. This is achieved by installing thick lead plates. Typical applications are golf carts, wheelchairs, people movers, scissor lifts and RVs. Starter batteries, in comparison, are made for maximum CCA (cold cranking amp). The battery maker obtains this by adding extra plates to get a large surface area for maximum conductivity. Capacity and deep cycling are less important for automotive because the battery is being recharged while driving. If continuously cycled, the thin lead plates of the starter battery would wear-down rather quickly. ...
-Rob
----- Rob Warnock
627 26th Avenue San Mateo, CA 94403 (650)572-2607