Trickle charging a battery

I have a diesel generator that uses a lead-acid 12V automotive battery for starting the engine. Normally, when in stand-by mode, the battery is charged from the grid. But I measured the voltage output and it was 12.6Volts, barely enough to start the diesel engine, especially in cold weather.

I contacted the generator manufacturer and they told me that the voltage output is normal :dubious: From what I know automotive batteries at 12.6 Volts are in need of recharging and when fully charged the voltage is 12.8

So I fashioned a trickle charger out of an old DC power supply that keeps the battery at about 13,8 Volts. Can I leave this charger indefinitely connected to the battery? Should I use a resistor to drop the voltage to a lower level?

A constant-voltage charger at 13.5v will keep the battery mostly charged, and shouldn’t wreck it. There are complete lead-acid charge control ICs, notably the Unitrode UC3906, which implement a temperature-compensated charger, which is really what you want.

Here is additional info on the care and feeding of Lead-acid batteries:

I just did a bit of research on lead-acid batteries for some hydrogen generation accident calcs. The way I understand it, the battery will begin to electrolyze the water into hydrogen and oxygen as soon as the battery is slightly charged. If the charger doesn’t kick off when the battery is fully charged, it will continue to electrolyze the water until there is none left in contact with the lead sheets.

Those are the two links I used for my calcs. Note the “Finish Rate” in the first one.

ETA: beowulff, figure 3 in your link is in direct conflict with the first sentence in your post.

From my link:

I keep this thread in my bookmarks. Lots of good car battery info given by Rick. He agrees with the generator manufacturer you contacted that a battery reading 12.6V is not in need of a recharge.

I think that a lead acid battery has a full cycle between about 11.8 and 12.8 volts. If it is really a deep cycle battery you can repeatedly use the whole range, or at least 80% of it. 12.8 is in great shape.

It behaves according to a diffusion equation, much as heating a thick slab of something from one surface does, the voltage and the temperature being analogous. Heating quickly causes the surface to get quite hot while the depth is still cold; that’s what the “surface charge” is about. Anyway, being at too low a voltage causes one kind of chemical damage, and changing too rapidly causes a second, and being held at too high a level causes a third, was my impression.

I saw that, but it still doesn’t jive with what figure 3 of that link says. Perhaps they mean two different things?

Also, I don’t understand how you can supply a higher voltage than the battery can charge at and not get off gassing when the battery is 100% charged. I can understand how it’d work with a charger that has a regulator/controller that turns itself off when it reaches the critical voltage, if you will, but I don’t understand how the same thing would work for a jury rigged charger that is a straight DC power supply without any type of cutoff.

Because 13.5 - 13.8 volts (depending on who you ask) is considered “full”, so the charger wont be delivering any current, except for the self-discharge and leakage current of the battery. Still, all non-sealed batteries need to be inspected and filled periodically. And BTW, how do you think automobile chargers work? They have about the least sophisticated charge management possible, and car batteries can last many years.

Are you talking about an external trickle charger? Don’t those stop charging when the red light switches to green? I have no idea how they’re regulated or controlled, but I’d like to find out. Or are you talking about an alternator charging the battery? I know it places a different load on the engine belt when it’s charging the battery, but I don’t know how that is controlled, either. Again, I’d like to find out, though.

I’m still not understanding why figure 3 of your link says 12.65 V is 100% charged, yet 13.5 - 13.8V is being claimed elsewhere.

I’m talking about car alternators. All they have is a voltage regulator, set at something like 14v. As far as the fully-charged voltage goes, I think that 12.65v is what you read after the battery has been disconnected from the charger. If the battery is connected, it’s OK to add another .15v/cell to maintain the charge.

You guys are making this way too technical. All he needs to do is get or make a trickle charger that keeps the battery at approximately the correct voltage. These are available everywhere and aren’t expensive at all.

I think you may be missing the other problems here.
1.) Lead acid batteries don’t like the cold and the their reaction time is slowed.
2.) Deisel fuel doesn’t like the cold and becomes thick, slowing down the flow through you fuel lines, filter, injectors, etc… as well as harder to ignite.
3.) Similarily, your glow plugs will take longer to heat up and as they do will require more power.
4.) It’s not the 12.6V that is not enough to start the engine, it’s the cranking(peak) amperage available after steps 1.) and 2.)

If it fails to start on the first attempt that could be all your battery can handle.

Some suggestions:
Keep the battery warm.
Keep the fuel warm, with a block heater, etc.
Check your glow plugs and replace or retrofit to more efficient, if necessary.

Personally, I would invest in a deep cycle gel battery rated at the cranking amps required for your generator. They are unaffected by cold, do not produce dangerous hydrogen gas when charging, and they hold a charge better.

Sorry it should say after steps 1-3.
Also, more importantly,

Give your battery a load test, you can take it to almost any garage and see if it is performing properly.

I guess this is the part I’m not understanding. Why the .15V/cell discrepancy?

That’s what we’re here for! :stuck_out_tongue:

Sparky, nice username/thread combo!

This is my opinion as well. Trickle chargers are made for this exact situation and are normally less than 30 dollars. Thirteen . eight is optimum charge an an automotive starting battery.

Well, I’m not clear on the chemistry going on, but my guess would be this: The instant a fully-charged cell is taken off charge, it reads 2.1 volts. It also immediately starts to discharge. The extra .15v provides enough energy to counter this self-discharge. Remember, that these batteries have a fairly flat discharge curve (take a look here:, so if you maintain the cell at it’s nominal “charged” voltage, it might actually only be 70% charged. Also, it’s probably better to have some slight amount of electrolysis going on, as opposed to having a partially-charged battery. Maintenance-free and SLA batteries are designed to deal with a small amount of electrolysis, but non-SLA batteries need to be inspected periodically, even when charged properly.

To reiterate what others have said, a fully charged lead acid battery has an open-circuit voltage of 2.11 V/cell. To quickly charge a battery that is depleted of energy, you can apply a constant voltage somewhere between 2.3 V and 2.45 V/cell. To “maintain” a lead acid battery on a continuous basis, you want to use a constant “float” voltage of 2.25 V/cell. When you remove the float charger from the battery, the battery goes back to 2.11 V/cell on its own. Keep in mind, however, that these voltages are for room temperature. As the ambient temperature goes up, these voltages should go down a little, and vice-versa.

If the generator is having a hard time starting, there could be a number of causes:

  1. Bad battery connection.
  2. Bad battery cable.
  3. Battery had too much internal resistance. (Bad battery.)
  4. Good but underrated battery.
  5. Bad starter.
  6. Engine oil has too much viscosity.

Is it a deep-cycle battery, perchance? If so, try using a regular battery. All else being equal, I’m pretty sure the latter has less internal resistance than the former.

Just a guess, But do what Crafter_Man said anyway, that the problem is a big lack of follwing directions and reading the book.

A big genny that uses a diesel engine and cost $$$$$ + more $$$$ is not likely to be built and sold with an inadequate starting system and I would prolly take a bet that the recommendations on maintance have not been followed.

Just cause it is a back up does not mean it can be safely ignored.

Maybe a bad part from the get go. Never know…


Depending on the engine. Consider installing a block heater.

Some engines need either an 8D in good condition or a block heater. some require both. And some require neither.

I just re-read this. If the battery is “charged from the grid”, does this mean it already has a charger on it? What kind of charger? Are you saying you are running a trickle charger at the same time?
You could be playing with fire…literally.