I am making a battery charger for a 45AH LEAD Acid Car Battery. That I will use with 100 W (1 A 100 V AC) UPS.

I need to know that what things I should keep in mind so that the battery charger charge the battery efficiently so as to extend its life. For now I only have this hint that a charger should stop charging the battery after it is fully charged.

What are the other features of a charger which extend battery life?

  • \$\begingroup\$ How about looking at how a cars alternator chargers the cars battery.. it not like its top secret technology! The same applies to your UPS. You should not charge lead acid to quickly, it can cause damage to the electro plates, otherwise make sure it is always topped up and don't over charge it. Also make sure not to keep your battery on cement/plain ground ;) keep it higher up. \$\endgroup\$
    – Piotr Kula
    Dec 14, 2011 at 17:02
  • \$\begingroup\$ @Piotr Kula, why not leave a battery on the concrete? \$\endgroup\$
    – Noji
    Nov 20, 2022 at 16:57

5 Answers 5


There are two important things to know about charging lead acid batteries.

    1. There is very very deep and arcane* magic involved, without the knowledge of you are doomed to miserable failure. (*But not necessarily dark).
    1. The magic has been very well investigated over 100+ years and there are exponents of the deep and arcane arts willing to share with you most (not all) of the secrets. If you are serious about your quest you should avail yourself of their expertise. If not, just take the red pill.

If you read what the masters say and follow it, your battery's life, and maybe your own as well, will be a long and successful one.

If you fail to diligently study the advice of the masters then you may get lucky. But, probably not.

Below I point you to the beginnings of your lead-acid journey, but as I do so I'll note:

  • Starting with "A Car Battery" [tm] is not the ideal way to meet your objectives. While it is possible to make a car battery last much longer than it would have without suitable care, it will not last anywhere near as long as batteries made for the task that you have in mind.

  • Car batteries may tend to contain stuff like antimony - because it makes them more resistant to physical abuse. Batteries that wish to live long and prosper (but that are less worried about mechanical abuse) may contain calcium because eg of its effect on self discharge rate .

  • It is unlikely that you will buy an off the shelf charger that really does what you want unless the sales pitch/blurb specs spell out that it does what you want in some detail. Most chargers are liable to do the basic charging thing and may be designed to "float" the battery. Any thought of equalization / topping cycles et al are usually far from their minds, as sales rather than performance are a priority.

  • Good does not have to be horrendously expensive (but can be). But, incredibly cheap is also usually "cheap and cheerful".

Here is a superb starting place:

How Lead Acid Batteries Work by Constantin Von Wentzell.

Note that his interest is "Marine Batteries" for use in his boat. The usage patterns of his application differ from yours.

  • His: fast charge, deep, often daily discharges, float when charged versus,

  • Yours: long term float with occasional deep discharges.(). Despite this he has much to say that is useful and valid.

If you read outside this page you will see that he started his investigations due to very bad advice given by a person whose positions in the industry would lead you to expect his advice to be good. But it isn't. ie "appeal to authority" is risky. Accept that an expert should know much, but also use your brain and consult multiple experts.

This site was referred to by a stack exchange member today - I has not seen it before [This is such a good page that it is plagiarized in many other places to help vend ads. If you see it elsewhere be sure not to buy the products advertised.)

Follow the arrows and read. Once suitably informed have a look at the next level up. He has links there to a number of other good resources.


All battery roads lead to Battery University.
They don't know everything, & not everything they say is correct - but you may never notice the flaws! :-). A good site. ( They note: "Sponsored by Cadex batteries".)

You should glance at their top level Learn about batteries page to get a feel for the scope of the site, and then leap into.
Charging lead acid .

DO note the subject menu on the left side of the page.
From the above page:

  • Charge in a well-ventilated area. Hydrogen gas generated during charging is explosive.

  • Choose the appropriate charge program for flooded, gel and AGM batteries. Check manufacturer’s specifications on recommended voltage thresholds.

  • Charge lead acid batteries after each use to prevent sulfation. Do not store on low charge.

  • The plates of flooded batteries must always be fully submerged in electrolyte. Fill battery with distilled or de-ionized water to cover the plates if low. Tap water may be acceptable in some regions. Never add electrolyte.

  • Fill water level to designated level after charging.
    Overfilling when the battery is empty can cause acid spillage.

  • Formation of gas bubbles in a flooded lead acid indicates that the battery is reaching full state-of-charge (hydrogen on negative plate and oxygen on positive plate).

  • Reduce float charge if the ambient temperature is higher than 29°C (85°F).

  • Do not allow a lead acid to freeze. An empty battery freezes sooner than one that is fully charged. Never charge a frozen battery.

  • Do not charge at temperatures above 49°C (120°F).


You could just buy a charger that claims to do all the things that these and other pages say a charger should do. But, you'd know a lot less :-).

Example of A plagiarising ad server ripping off this sites copyright material. And again more plagiarism and yet again.

Better: A site acting as if it is a search engine and accumulating related sites to try and sell ads - actually useful - Performs a service and no copyright infringement.

  • \$\begingroup\$ Thanks for such a great and well described answer. You said that Lead Acid Batteries are not a good choice for this purpose. Then which kind of batteries will be more useful for a UPS. \$\endgroup\$ Dec 15, 2011 at 14:16
  • \$\begingroup\$ @TheNoble-Coder if you are looking for UPS you want to go for Deep-Cycle lead acid as opposed to Car Battery [TM] lead acid. Most of the risks about hydrogen can be reduced by using a sealed gel variety. en.wikipedia.org/wiki/Deep_cycle_battery en.wikipedia.org/wiki/Gel_battery#Gel_battery \$\endgroup\$
    – klonq
    Dec 15, 2011 at 14:26
  • 1
    \$\begingroup\$ I said car batteries are not ideal, as opposed to lead acid. Car batteries are a subset of lead acid that aim at lowest cost while starting in cold mornings and usually being floated in a somewhat hostile environment. Self discharge is less important than robustness. They are not deep cycle designed and will be destroyed rapidly by deep cycling. In some cases they will be good enough for UPS use - but othersare designed with this in mind. \$\endgroup\$
    – Russell McMahon
    Dec 16, 2011 at 2:07
  • \$\begingroup\$ @Pro Backup [tm] - I have rolled my answer back to its prior version as you made many gratuitous changes which were unnecessary and this made it impossible to sort through the useful changes that you had made and keep them while undoing your stylistic and opinion based major modifications. I do not know why you feel qualified to hack about with answers when you have only answered 3 here in 1 year plus with a total of 2 upvotes. I suggest you get some "site milage" before majorly hacking about with other people's posts. \$\endgroup\$
    – Russell McMahon
    Apr 13, 2015 at 18:11
  • 1
    \$\begingroup\$ I agree with what you said, but there is one good thing about car battery [tm.] You can pick them up secondhand for nothing. They won't be reliable enough for a data centre, but they will be reliable enough for a home backup if you live in a developing country. OP is from Pakistan. \$\endgroup\$ Nov 26, 2015 at 20:15

Basically, the key to extending the service life of any kind of rechargeable battery is to identify conditions which can cause premature failure and ensure that they don't occur. Overcharging and overdischarging can both cause damage to lead-acid batteries. Absolute mechanical stillness can also cause problems. If a battery sits absolutely motionless for a long time, stratification of the electrolyte may occur, with the heavier acid settling toward the bottom of the battery and the lighter water floating to the top. Forcing or drawing enough current into or out of it to physically agitate the electrolyte may prevent this from occurring. Note that the act of forcing or drawing enough current through the battery to agitate the electrolyte may cause some wear in and of itself, and that if there is enough other agitation of the electrolyte (e.g. from naturally-occurring vibration), forcing or drawing the current may not accomplish any useful purpose.

  • \$\begingroup\$ I'd like to add that lead-acid batteries are "consumable" devices. They convert chemical energy into electrical energy when discharging, and electrical energy into chemical energy when charging. However, some of the battery is destroyed in the process, and nothing can be done to eliminate that. The addition of antimony or calcium may make one battery more rugged or self-discharge less, but there is always a trade-off. Eventually it will "wear out" and refuse to function. All a "deep cycle" battery means, is that it's lead plates are thicker, so it can withstand more big dis/recharge cycles. \$\endgroup\$
    – rdtsc
    May 15, 2015 at 2:47
  • \$\begingroup\$ @rdtsc: It's true that even under ideal usage conditions lead acid batteries will eventually deteriorate to the point of uselessness, but that is true of almost anything. Adverse conditions can expedite deterioration by orders of magnitude, so while proper treatment won't make batteries last forever, it will avoid needless premature death. \$\endgroup\$
    – supercat
    May 15, 2015 at 15:11

You don't want to stop charging completely once the battery is charged. You still want to keep some kind of float charge going even if the battery is fully charged.


In addition to what others have said, i would either use a charger with a built-in desulphator or an external desulphator. Batteries not being regularly discharged have a tendency to sulphate.


The answer to your question can be relatively easily answered given what manufacturers say about their own battery. They thoroughly dictate how their battery should be handled in different scenario that is: charging / float voltage depending on the INTERNAL battery temperature & battery reconditioning (desulfating). Every charger I've seen (the most popular) only consider the AMBIENT temperature and empirical formulae to approximate the correct charging/float voltage. During charge (especially if the current is high), the battery will heat and the recommended voltage goes down so it's possible for a "smart" charger to overcharge a battery due to that but also when the battery is brought from the exterior (hot) to a cooler place to be charged. Batteries are quite massive and they can store a lot of heat.

The issue with "smart" charger is that they think they're too smart, i.e., they don't let the user define what the manufacturer actually recommend. I think the reason is quite simple as it would probably cost more to design such charger than having it simply controlled by a cheapo micro-controller. This is especially true for desulfating or reconditioning. There are many way to do it but some battery (like dry ones) and cell composition (elements) need specific procedure to do it. Some charger use rapid pulse but it all comes down to forcing higher voltage to break down the sulfate layer.


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