What I've learned:

A: UPSs generally use sealed lead acid batteries, which use a slightly higher voltage to charge. Using an auto battery will result in the battery being in a slight state of overcharge requiring period topping up.

B: UPS inverters are not designed with cooling in mind to run at rated capacity for more than the nominal 15 minutes. Of course oversizing will get you longer run time and a lower percentage of rated capacity. Cooling could be as simple as a thermostat and a small 12v fan, or an oversized box so it had more surface area. (Merit in making it roughly the same footprint as a car battery, with legs that straddle the bumpy bits on the top of the battery)

C: One forum stated that a car battery cannot provide the needed power, averaging only about 150 watts (12v @13 amps) This may be the C/5 or C/20 discharge. A typical auto battery stores about 100 amp hours or about 1.2 kWh. 150 W corresponds to about an 8 hour full discharge or about 4 hours without damaging the battery.

C: A car battery is not robust against repeated flat line discharge.

My experience has been that conventional backup power supplies such as APC™ and Cyber Power™ have MUCH smaller batteries than a car battery, will typically handle their rated power for 15 minutes. Even used infrequently they seldom last more than 2-3 years before they start to complain and want battery replacements, which are much more more expensive per pound than auto batteries. By comparison, the car battery that supplies my backup sump pump typically lasts 5-7 years between replacements.

Seems to me that a unit designed to use an auto battery, while not great for repeated long power outages, would be fine for the frequent short outages that many rural dwellers encounter. Properly designed, it may be possible to switch batteries on the fly using a double pole make before break. This would result in a short time where the fully charged battery would attempt to charge the used battery.

BUT so far I've been unable to 'divorce' the charge/inverter/filter function from the energy storage function in a consumer product. I'd like to understand why not.

Edit: So additional background.

The replacement battery for an APC Back-UPS Pro 1500 costs $US130 and weighs 12.8 lbs. After market ones run about 2/3 of this.

A Silverado battery at Costco costs US 116, and weighs 37 lbs. My experience with my truck suggests that a battery can be run to zero a dozen or so times and still hold enough charge to start the truck in winter.

I have a 12 v sump pump with an open cell marine battery on it. The house has an overall air change time of under 3 hours. Gas accumulation seems unlikely. As to acid droples (referencing another post, it has a plastic box, with a non-tight lid, and the top of the battery is sprinkled with baking soda. The soda is my addition. The product is intended for not very well ventilated basements.

  • $\begingroup$ Your title question does not match what you ask at the end of the post. Focus on what you really mean to ask. $\endgroup$
    – Solar Mike
    Commented Jun 11 at 9:54
  • $\begingroup$ I've reworded the title. Better match? $\endgroup$ Commented Jun 11 at 12:55

3 Answers 3


Conventional flooded lead acid batteries used in automobiles and other vehicles enjoy an airflow within the compartment in which they are mounted. This is critical, as the batteries will generate hydrogen gas as they are charged. Overcharging will create even more volume of this gas. It is also expected that the charging process will create bubbles in the electrolyte. When the bubbles pop, some of the acid within the cells are aerosolized and dispersed in the area.

Valve regulated sealed lead acid batteries will vent under pressure if overcharged, but in normal operation there is no danger of hydrogen gas, nor of dispersal of acid vapor. The details of the battery construction can be found at a fairly clear web site, electricity-magnetism.org and covers the various chemistries of this and other types of batteries.

Within the linked site, one can also find a reference that VRSLA batteries do not leak, allowing orientation of convenience rather than a strong requirement that the battery remain upright. One does not place a flooded lead acid battery on its side, for obvious reasons.

It is not typically necessary to maintain battery backup while switching out batteries. The battery is not in use unless the power has failed. One should consider to move the protected devices to another source temporarily or to perform the battery swap while the UPS is energized. Unless the power fails during this swap, there will be no current drawn from the battery.

Regarding extra capacity: one of my UPS units cautions explicitly in the manual (from years ago) to maintain existing ratings. A longer rated battery will certainly allow longer run time but create excessive temperatures, as you've noted.

It's expected that a lead acid battery lifespan is approximately three years, sometimes shorter in hotter or colder temperatures.

  • $\begingroup$ As to switching batteries: The intent was to be able to switch batteries while the mains were dead, without interrupting the power to the device being backed up. E.g. if you had an important system, and a plan for a 4 hour outage, that you found was going to be 12 hours. I don't actually anticipate needing this feature, but it would kind of neat to have. $\endgroup$ Commented Jun 11 at 13:18
  • $\begingroup$ I have a 12v battery in my basement on a charge maintenance unit. Whole thing -- battery, pump, charger, and plastic box came from my hardware store. H2 hazard must be fairly small for a single battery in a normally ventilated space. $\endgroup$ Commented Jun 11 at 13:25

For less cost and less weight, you can choose to optimize for fast discharge, or for better deep-discharge battery life.

Starter batteries are designed with large plate area and small plate and electrolyte thickness, to get high current for short periods at optimal cost.

Deep Discharge batteries are designed with smaller plate area, thicker plates, and wider electrolyte layers, so they are unable to provide very high discharge currents, but are less likely to suffer damaging degradation on each deep charge/discharge cycle.

For more money and more weight, you can just make bigger and bigger batteries. A bigger battery weighs more and costs more, but can provide higher discharge current and allow higher charge, and at shallow charge/discharge will last for more charge/discharge cycles.


A car battery is designed to provide high current burst to turn over an engine. Then get charged to full while the engine is running.

Whereas a backup power supply needs to provide lower steady current over several minutes. This requires a different design of the electrodes.

BUT so far I've been unable to 'divorce' the charge/inverter/filter function from the energy storage function in a consumer product. I'd like to understand why not.

Companies like to make money, therefore putting a premium on the proprietary replacement parts that wear out/run out (aka the battery) is the goto business decision.

You can find stand-along inverters that take in a stable DC power and give out a AC output. You will also be able to find battery management devices that will take in grid or solar power to charge the battery.

  • $\begingroup$ Not quite what I asked. Often the market will provide multiple solutions that vary in price, longevity, ease of maintenance, ease of installation. This seems to be a viable solution that would be lower cost, lower maintenance (auto batteries are easier to find) more difficult installation. (heavier, multiple components) Stand alone inverters won't charge the battery, and won't switch over automatically. $\endgroup$ Commented Jun 11 at 13:23

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