float charging / maintaining / desulfating a group of different batteries

Discussion in 'General Electronics Chat' started by ksoggs, Jul 5, 2010.

  1. ksoggs

    Thread Starter New Member

    Jul 4, 2010
    I have several lead acid batteries that i would like to keep charged year round when not in use. I have pulled them all out of their respected homes....

    What i was wondering is if i can charge them all up, and top them all off, and then bank them all together in parallell and maintain them all with a single float charger and a single desulfator.

    The batteries vary from HUGE deepcycle batteries weighing 105 pounds each, to standard car batteries, to motorcycle and tractor, to small 7ah sla's.....

    some are Sealed, some are flooded, some maint-free flooded, some are AGM, etc etc.... But all seem to have a resting voltage of about 12.9-13.1 and it appears they are all comfy with a float of 13.8.

    So as long as they are all topped off, and in good shape... is there any reason i should not put them in a bank and charge them all together? it would sure be easier than buying 10 different float chargers.
  2. ksoggs

    Thread Starter New Member

    Jul 4, 2010
    just a note... the bank would ONLY be for maintaining... i know i would never want to discharge a bank with such assorted sizes and varieties of batteries....

    But i dont see why i cant float charge and desulphate them all at once assuming they are all brought to 12.9v and are all comfy with a 13.8v float before joining the bank.

    I am just more curious about possible issues, or safety concearns, or anything that effects the life of the batteries... obviously i am trying to do this to help them last longer. So last thing i want is to fry them or hurt them.
  3. SgtWookie


    Jul 17, 2007
    I advise against connecting them all in parallel. When one of them eventually develops a shorted cell (a very common failure mode amongst lead-acid batteries) then it will cause the entire bank of batteries to become discharged (or very high power consumption from the charger), while the shorted cell emits hydrogen and oxygen gases.

    Just as an example, our local Vietnam War Museum has a number of retired military vehicles that are maintained in operating condition, and are used in parades.

    One of them is a USMC 5-ton truck. This truck has four very large 6TMF lead-acid batteries that are connected in series-parallel; it is a 24v (28v) system. The batteries are kept charged during non-use periods via a solar-powered charger.

    One of the batteries developed a shorted cell. This caused the other battery in series to be overcharged, and the two batteries in the other series string to become discharged below normal, resulting in the early failure of the other three batteries. All had to be replaced at a cost of $180 per battery.

    What you need is an intelligent battery maintainer that is capable of selecting each battery in turn in a "round robin", measuring it's static voltage (and preferably internal temperature) and then deciding upon the appropriate action to take; applying a float voltage, equalization charge (deliberate overcharge for desulphation and stirring the electrolyte), warning of problems like a shorted cell, etc.

    Of course, this would be a fairly complex project, and would require the use of a microcontroller and a switching supply for the best efficiency at a reasonably low part count. At a minimum, the uC will need to be told what type of battery each station has connected to it; ie: SLA, gel, AGM, deep-cycle, automotive, marine. AH rating could be determined via a load test, but it would be less complex if the AH rating were also entered.

    SLA batteries generally need a higher float charge than other lead-acid battery types. Gel cells cannot be "equalized"; if a deliberate overcharge is attempted, bubbles will form in the gel that will permanently reduce the battery capacity.

    There is a LOT to learn about the proper care of batteries. This site has lots of information on various battery types:

    Once you digest that information, you need to get the specifics on each battery that you have on hand. The best way to get the particulars is to get the manufacturer's datasheet for the particular model of each, and read them.

    One item of note is the negative temperature coefficient of lead-acid batteries. The general formula is -3mV*number_of_cells/1°C, with the standard temperature given at 25°C (77°F) - so with 12v batteries, it's -18mV per 1°C deviation from 25°C. Many chargers are not temperature compensated, which leads to overcharging at elevated temperatures and undercharging at low temperatures.

    With some exceptions (notably AGM batteries), rapid charging leads to elevated core temperatures. An increase of 30°F (16.7C) decreases battery life by 2/3 due to the increased chemical activity.

    It takes a LONG time to cool the core of a battery once it has been heated.

    At any rate, I've rambled along for awhile now. Consider this just an introduction to some of the problems you're facing, and methods of gathering information.

    I suggest that you read through the Battery University site, and then locate the datasheets for ALL of the batteries that you need to maintain. In order to make a good decision, all of the basic information needs to be considered.

    I've actually been meaning to do something like this for awhile now. The Museum simply can't afford to be replacing batteries every year or two. Fortunately, all of the batteries we need to maintain are similar types, if not the same AH capacity.
  4. ksoggs

    Thread Starter New Member

    Jul 4, 2010
    I have already nearly memorized most of the SLA info on battery-university... great site, one of the sites that really got me hooked on lead acid batteries.

    most of the batteries are just elcheapo over the counter types... Die Hard car batteries... APC SLA UPS batteries... Only good batteries i have are my AGM SLA Deep cycles... I already have the data sheet for them. and they like a 13.7-13.8 float. And then there is my harley battery, it sat for too long and is pretty sulfated. It charges up fine, but it doesnt last long at all under load.

    I have a few cheapo harbor freight wall-wart style float chargers.... and i have one "batteryMinder charger/maintainer/desulphator" and then i have one "solar converters BC-2 desulphator"

    The BC-2 has a very strong pulse strength, and is adjustable. So it works great for the big guys and to really knock out sulphation in a hurry. The batteryMinder is just a tiny 1.3a multistage charger with a desulphator included.. it is no where near as powerful as the BC-2.

    But both the BC-2 and BatteryMinder say they are capable of servicing banks of batteries, as long as they are all fully charged and relatively healthy before "banking"

    But that is about all they say.

    neither one has settings for different chemistries, or anything. just basic "hook it up and walk away"

    I would really like to keep them all on a float charge, and would really like to leave them connected to some type of desulphation device...

    but i cant afford to buy chargers and de-sulphators for all of them. it would be cheaper to just replace an occasional battery.

    I know there are always dangers of failure in any single battery, and even more so when you bank them...

    But in general, outside of a physical failure of one of the batteries, or a defective charger, or what not.... Assuming nothing "dies" on me....

    is there anything to worry about in doing this? Like if i were to disconnect and inspect each battery every month or so... to prevent a bad battery taking out the whole bank...

    is there a danger of over-de-sulphating the smaller batterys? or could any battery be hurt by a 13.8v float? (i know car batteries spend most of their life sitting at over 14v's)

    So apart from major failure... what practical concearns would there be.....

    here are some of the documents i have.

    my battery's data sheet.

    BC-2 instructions.

    batteryminder instructions
    http://www.thebatteryminder.com/a_add/manuals/Manual 12112 _2004_for_web.pdf

    and then some photos of what i am working with here.
  5. SgtWookie


    Jul 17, 2007
    The value of "desulphator" circuits is debatable. They do slowly remove sulphation from the plates, but they do not stir the electrolyte. You would either need to mechanically agitate the cell (which would very likely cause other problems), or apply a deliberate overcharge for a controlled period of time (equalization) which is the standard maintenance method.

    It sounds like you want a blessing to connect them all in parallel anyway. Hey, they are your batteries - you can do with them what you wish. I did start off mentioning (in more words) that it's risky to wire them all in parallel, as any cell can short at any time, particularly with used batteries.

    For example, if one cell shorted right after you performed a monthly check, by the time you did your next check, the batteries would all be dead - or you would have a very large electric bill from the float charger trying to keep up.