High capacity lead acid charging question

Hi all,

I am about to install a pure sine inverter into my van as part of a conversion. Have used them in the past connect to a second battery that is charged by using a simply split/charge relay arrangement. This time however I hoped to do it in a much neater fashion by using voltage/current sensing of the alternator output level. All this is connected to a 16F877 (with LCD display) that does the monitoring and switching of the charge relay with added relay contact change over sensing. Not sure of the reasons but simple split charger circuits seem to suffer premature failure.

My question is, if I were to monitor (not control) the current flowing into the second battery whilst charging would this give me a good indication of charge state. I believe that if I monitor the charge current and it drops to 1/10th of rated battery Ah, one could assume that the battery is charged (with 12.65V terminal)????

The reason for this I have know in the past secondary battery's gas-off (potentially explosive) if constantly charged when not required. My reason for wanting to do this is that for the majority of the time the second battery will not be loaded so I wish to avoid unnecessaily over-charging it.

I realise that I could just charge it for n time, disconnect it, apply a load for n time then retest to determine battery output but for this project this is a little overkill.

Any advice would be appreciated!!!!!

Many thanks,
Andrew

I think you would have to be careful of what would happen in a fault condition. Say one cell shorted, you would probably always have a high charging current and gassing of the remaining five cells. The fully charged voltage of a lead-acid battery is also temperature dependent - my solar PV charge controller has voltage, current and temperature sensing.

ermine said:

I think you would have to be careful of what would happen in a fault condition. Say one cell shorted, you would probably always have a high charging current and gassing of the remaining five cells. The fully charged voltage of a lead-acid battery is also temperature dependent - my solar PV charge controller has voltage, current and temperature sensing.

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There is always one I could add a temp sensor at the terminal quite easily; resolves the temp issue. The problem I see is that my thing sits inbetween crude and the proper methods of charging. The only saving grace is that my thing checks the level prior to initially charging so it would be hoped that the the terminal voltage will be way down with a shorted cell. The problem would come if the cell shorted during a long run. For that reason the battery is under the floor/outside but wrapped to insulate it. Thankyou for the hint though regarding shorted cell.

I suppose if I monitor the voltage with respect to terminal temperature one could assume to battery is pretty ok.

Lead acid batteries are normally charged with constant current (i.e. 500 mA) until the voltage reaches certain level (this is called Bulk-charge).
After bulk charge the charging continues with constant voltage until the current drops to a certain level (this is called Over-charge).
Finally the battery charge is maintained by applying a float charge voltage.. or the battery is disconnected from the charger.

The battery manufacturer should be able to give you all the voltage levels and current limits you need to know to properly charge the battery. Many batteries have these values printed somewhere in the battery casing.

You don't have to charge the battery with constant current. Just don't go over the current and voltage limits. And don't keep the battery at the Over-charge stage if it doesn't take in current... or takes too much.. or doesn't stop taking in current.

I suggest that you consider floating the battery. I have had a stationary battery that supports a telephone system on float for 8 years without a failure. The proper float voltage is 2.25 volts per cell for a lead acid battery. The trick is to hold the float voltage steady in spite of changes in load or source voltage.

Under idle conditions, only enough power goes into the battery to make up for internal losses and self discharge. When the engine is running and there is demand, the engine provides all of the power to the load. The battery only provides power when the engine is stopped and there is load. By limiting the amount of power available from the generator you also prevent the catastrophic failure of a shorted cell since the input voltage would load down. You could also use a current fold-back circuit to protect from a short in either the battery or the load. An open cell fails with no current in or out of the battery so no special precautions are required.

Thankyou for that. The website for the battery manfacturer does not have a 'detailed specification', I will email them and hope to obtain it. I have a question....

Does the strict charging regime for a lead acid or I suppose any battery apply only when charged in isolation (out of application). I ask only as the charging system in a vehicle is very crude when dealing with the main/starting battery. Are the charging rules in terms of voltage/current apply when the battery is actually in service??? I understand that a massive drain is placed on the battery during cranking so lots of current is good.

Ya, I also suggest that you consider floating the battery. I have had a stationary battery that supports a telephone system on float for 5 years without a failure.

I'm doing something similar to maintain the flooded-cell deep-cycle lead-acid battery in my RV (travel trailer). I have a modern switch-mode 120Vac 35A power supply/charger which has screw terminals to select two different preset charging voltages: 13.2V for float, and 14.25V for charging. When I bought the trailer, the screw terminals were open, meaning that the battery only got the float voltage. I made a circuit that automatically recharges the battery at the 14.2V level (by shorting the screw terminals) until the charging current drops to under 1A, and then automatically reverts to the float voltage (by opening the connection between the terminals).

The circuit is basically a flip-flop which gets set if the charging current is >3A, and the flip-flop gets cleared as the charging current drops below ~1A as the battery reaches full charge. The output of the flip-flop switches the power supply between its two preset voltages.

Imagine the following use model: Battery is fully charged to begin with. Trailer is used for a weekend in the mountains. Battery powers lights, water pump, radio, etc, so when I return home, and plug the trailer into the AC line, the power supply is trying to put out 13.2V, but the supply actually goes into its 30A current-limited mode, where the battery holds down the system voltage as the battery accumulates charge.

The initial high charging current sets my flip-flop, the supply stays in the constant-current mode until the battery voltage rises to 14.25V, at which point the supply automatically goes into constant-voltage mode, holding the voltage constant at 14.25V, while the charging current begins decreasing.

My circuit is watching the decreasing charging current, and when the charging current drops below ~1A, the flip-flop clears, causing the power supply to switch to its other preset level of 13.2V, which is appropriate for floating the battery 24/7. I leave the trailer plugged in while it is parked at home.

I use a ZXCT1009 high-side current monitor to sense the charging current. Its output sets/clears the flip-flop, which is basically an op-amp used a comparator (with a lot of positive feedback (hysteresis). You could implement the current sensing the same way I did, but use your PIC and its ADC as the "flip-flop". I'll post the schematic if you want it...

Hello Mike,

Sounds more or less the same. If you could post the schematic that would be sincerely appreciated!!!

Many thanks,
Andrew

burgerfeet said:

The problem would come if the cell shorted during a long run. For that reason the battery is under the floor/outside but wrapped to insulate it.

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Why do you insulate the battery? Batterys last longer when they say cool.

burgerfeet said:

The problem would come if the cell shorted during a long run. For that reason the battery is under the floor/outside but wrapped to insulate it.

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I agree with crutschow to question you for wrapping the battery. Lead acid batteries should be kept in a well ventilated area to get rid of the gases (and keep the battery cool). Of course you should also keep sparks away.. maybe this is the reason for wrapping, but I don't think it is a good idea.

I say insulated, it has a jacket on as fitted to a number of vans. Probably a thin layer of protection would be a better description We had an issue with batteries freezing locally and nationally at the beginning of the year during our major cold snap, most pobably due to poor charge. Granted, keeping them cool, they last much longer.

Here you go. All of the stuff on the left side of the schematic simulates what was already in the trailer, with the exception of R2. The time scale in the simulation is obviously compressed...

burgerfeet said:

...We had an issue with batteries freezing locally and nationally at the beginning of the year during our major cold snap, most pobably due to poor charge. ...

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If a battery is kept on a float charger, i.e. the Specific Gravity is kept at the fully-charged level, the battery will not freeze unless the temperature gets below about -30degC. Wrapping it in a blanket will not help if the ambient temp drops well below freezing for more than a few hours...

It may last longer at low temperatures, but that is caused by retarded chemical activity. Insulating the battery enough to keep the temperature around 20° c. will allow you to pull the rated power out of the battery. When it is warm out, remove the blanket.