I have a computer and LCD with DC power supplies that I'm going to run on 12V golf cart or wheelchair batteries. My best estimate is that I'll need about 50AHrs (@12V) of deliverable energy. The problem is that batteries terminal voltage decreases as it discharges. I'm not sure how sensitive the input voltage is on my PC just yet, but I know that at some point it will fail.

Does anyone know if batteries in parallel might do better with holding their voltage? What about using capacitors?

I can get a 75AHr battery that may work fine. Do you think 2 40AHr batteries might work better for me?

TIA,
Nathan

 

I'm not sure what your actual application is, so its hard to make a balanced recommendation. However to begin with, putting batteries on parallel will provide more deliverable current but it doesn't change the battery curve. The battery curve is a characteristic of the battery which describes how the voltage drops off under load. This is a characteristic of the particular battery chemistry. Different chemistry results in a different curve shape. As the battery ages, the curve also changes (you lose capacity).

Having two 40 AHr batteries in parallel should be essentially the same as one 80 AHr battery, provided the batteries are of the same type. Cell voltage in a battery is determined by the chemistry. The number of cells in series determines the output voltage. Once these are set, the main variable left is basically the reaction area. Putting two batteries in parallel is the same as one bigger battery. Having a bigger battery means that for any given load you are proceeding along the curve towards discharge more slowly, but you haven't changed anything fundamental about the chemistry. So for any given load a larger battery system means it lasts longer, but the curve still has the same shape.

If the PC and LCD run from DC power (you haven't mentioned an inverter), they should list a minimum and maximum voltages. You then need to plot these (and the current draw) against the battery curve of whatever battery you plan to use. An electronic converter or inverter lets you extract more from the battery capacity before the final output voltage drops too low.

A (super) capacitor is probably not what you want, because the charge drops along an exponential curve. You want something that maintains a curve that remains as flat as possible until it drops off when nearly discharged. Super-caps are good for sudden surges of power, or for providing a small trickle to maintain volatile RAM.

If this is a vehicle or marine application, you might look at using an off the shelf vehicle power system. Try http://www.xantrex.com for more information on these. These can work with external batteries, or you can get a self contained system.

If this is actually a UPS type application, then why not just use a large UPS? If you use a large enough UPS, you should be able to get enough run time from it. They also have the advantage that the computer can monitor the state of the battery via a UPS connection (assuming the computer and software can do this).

If the batteries are part of a vehicle power system, you might need to look at protecting the computer from "load dumps". You can get large swings in voltages (up and down) as loads are removed or added. These can damage electronics if the design does not take these into account. In automotive applications, a load dump can result in 18 volts or even higher on a nominally 12 volt system. If the power system is only supplying the computer and UPS though, this shouldn't be a problem.

 

I would go for the single battery with the appropriate capacity unless you have the means and desire to get matched sets of individual batteries making up the total capacity. If you don't, you may find that one will discharge well before the other creating problems. More components... more chance of failure.

Lead acid types certainly have a sloping discharge time/voltage curve that you will have to deal with. Perhaps a small circuit to warn of low voltage before your system craps out? Many of the newer battery technologies ($$$$$) have a much flatter discharge/voltage curves. But then you still have to monitor the consumed power before you go over the "edge of the cliff" and the voltage drops off dramatically in a very short period of time. Guess there ain't no free lunch.

Gerald Beaudoin

 

Be careful with the Amp-hour rating. You might expect a 50 Ah battery will supply 1 Ampere for 50 hours. It doesn't work that way, unless you have a converter that can extract energy from the battery all the way down to zero Volts. Lead-acid batteries are fully charged at 13.8 Volts. Electronic equipment expecting 12 Volts will not function when the battery voltage drops below 10.5 Volts. A DC to DC converter can maintain a constant voltage to your computer while the battery voltage drops.

Yes, you can place batteries in parallel to add capacity. I have two gel cell batteries powering my ham radio. I charge the batteries using solar
panels, and a charge controller to prevent over charging. During discharge, the two batteries discharge at the same rate. The only caution here is to replace all batteries at the same time. The life cycle for large batteries depends on the number of discharge cycles. Let's say a battery has a lifetime of 750 discharge cycles. If all batteries in the system enter service with the full lifetime ahead of them, they will discharge at the same rate. If one battery is half way through its lifetime, then the entire system will fail when that one battery fails.

Ken

 

Thanks Michael - that all makes sense. I think I'm opting toward a wheelchair battery. A UPS doesn't make sense because the LCD and computer both have DC power supplies.

 

Nathan,

It is possible to have two or more batteries in parallel using diodes which removes the potential (no pun intended) of one feeding the other - while supplying local currents.

The diodes allow the batteries to supply the current as long as the power source is available (note--diodes will probably eat up 0.7 to 1.4 volts). The charging of the batteries is another issue.

Have fun.

Dennis

 

For this application look into using Schottky diodes. The forward voltage drop can be much lower. <.5 Vdc.

Regards
cww

 

Definitely check the amp-hour rating. On the battery's datasheet, it should give the amp-hour rating at various discharge rates. Look at the one hour rate to get a better idea of the battery's true capacity.

Many (but CERTAINLY not all) DC 12v devices are happy from about 10.8 volts on up to around 14 or 15. Computer equipment is a little more picky, but it still should be fine. Check the ratings.

You could always get a switching 12v DC power supply (DC-DC converter) to connect between the batteries and the computer. This will operate in boost or buck-boost mode (depending on the controller), to stabilize the voltage at 12 volts for as long as possible.

Just the batteries would likely be fine, though.