charging battery

[trickyh]

With small to medium gel cells you do not want to push more than 1/2 amp into them ever, as they cannot take the current like a wet battery can. Even a car battery charger will damage it.

Invest in a proper gel cell charger from cpc etc, they're not expensive.

http://cpc.farnell.com/ansmann/bca120-350/charger-lead-acid-12v-350mah/dp/BT06035

 

[ericmark]

Although you can get these

designed to charge a second battery likely they are not required.

So let me look at split charging and pros and cons of second battery charging.

Two main things.
1) Not to run down donor battery.
2) Not to overload wiring between the two batteries.

So the aim is to ensure when you crank the engine the second battery is disconnected, or it can burn out the wiring or blow the fuse at least, and then connect the two batteries together when the engine is running with as little volt drop as you can. Likely the cig lighter is disconnected while cranking engine, and also disconnected with ignition off, so likely you can simply plug second battery in with no special relays or diodes, if only connected while ignition is on plus disconnected on cranking.

However as battery size increases so it gets more and more complex.

Where the alternator AVR has Hi/Med/Lo settings moving to Hi and using a standard Lucas blocking diode was an option, however with most cars you don't have that option, the Schottky diode does reduce the volt drop, and is use with narrow boats, but here you have all the batteries together and engine running 6 hours a day. In real terms the split charging relay does a better job, less volt drop. Often the relay is switched by the ignition warning light supply from the alternator.

Today however the car charging system has evolved, in some cases the car stops charging when on power, and only charges on over run or when battery very low. As a caravan user we found over the years split charging has become harder and harder and also the demand on the battery has got higher and higher. So using an inverter has become more and more popular.

The battery charger can have an output voltage between 13.2 and 16 volt, the latter was a two bobbin regulator and rare today, at 13.2 volt looking at 24 hours to charge from fully discharged, many cars go up to 13.8 volt so average is around 13.5 volt, which for most cars is ample. However when time is limited, like when charging a fork lift or milk float then 14.8 volt is more normal until 90% charged, last 10% is slower at 13.4 volt, with valve regulated lead acid (VRLA) 14.4 volt is more normal again dropping to 13.2 volt for last 10%. The way the charger works out when down to last 10% is charge current, this depends on battery size, normally around 1/20 to 1/40th of the amp hour rate. So for 330 Ah of sons boat then 8A was the trigger current, however there is a problem when battery is used at the same time as being charged. So there is another method, pulse charging is often used with wind and solar charging, it is also used with narrow boats, not unusual to have 2 x 120A alternators feeding an inverter which allows the battery to be recharged in just 10 hours. However since the narrow boat is often only travelling for 6 hours a day, the batteries have to be large enough to only become 50% discharged.

Now you are looking at a lot of money this

30 amp version is £250 for 60A it jumps to £350 note alternator to battery chargers are not the same as battery to battery chargers the former are cheaper, see here but the main factor is speed of charging.

Although you can get these chargers in the case of the small VRLA battery step one is check if anything is required. So three tests.
1) No power to cig lighter with ign off.
2) No power to cig lighter when cranking.
3) The voltage is not over 13.8 volt.
Remember a car does not normally have a valve regulated glass mat battery fitted, the charge voltage can therefore be higher, even with 13.8 volt one needs to ensure it's not left on charge too long, a standard diode has around 0.6 volt drop, so if the voltage is too high, then add a diode to drop voltage.

 

[trickyh]

This little puppy will happily charge your SMALL lead acid all day from a car battery whether the engines running or not. Small gel cells are typically used in alarm panels and are designed for LOW charge and discharge.Believe me if you try to stick more than 1 amp into it, you will make it go pop or seriously shorten its life span.

http://www.mardave.co.uk/all-parts/85-284-detail

 

[ericmark]

I think the valve regulated lead acid battery that uses the glass mat system to hold the electrolyte in place (there is also a gel VRLA) has some serious short coming. I have some VRLA batteries in a mobility scooter which are quite large, and in spite some bad maintenance over the years are still going strong after 10 years of use. These batteries are charged with a stage charger. However the wheel chair batteries failed far quicker, and on the third set with the chair lift, house alarms seem to struggle to get more than 2 years.

I personally blame the charging, from an apprentice I was always told the float voltage should be 13.2 on a 12 volt battery, but often the charger gives it between 13.4 and 13.8 volt, it was so easy to make a 13.2 volt regulator, the standard 7812 clip gives 12 volt and a red LED has 1.2 volt across it, so putting a red LED between the Com on the 7812 and real Com lifted the volts to 13.2 volt and this was the standard way to power a CB radio. It was also a good charger for small batteries, hence that figure 13.2 was ingrained in my memory.

Over float voltage then current does need control, and also some method to disconnect should a cell go short circuit, however the voltage is the main control not the current. When we talk of three stage charging in real terms it's two stage, the charger gives maximum current until a pre-set voltage is hit, but there is not really a change over from current to voltage regulated, the current is simply the maximum the charger can deliver. The real special bit is once that voltage regulation takes over, as the battery becomes charged so the current drops, we can measure this current and use it as an indication on state of charge, what is really important with valve regulated lead acid is that it is not over charged, there is simply no way to replace any water, so too things, one the fast rate is reduced from 14.8 volt to 14.4 volt, many stage chargers have dip switches to change this voltage, and the second is we must reduce charge rate as it approaches fully charged, most stage chargers are for a range of battery sizes, for example 100 to 200 amp hour, if the battery is too small, then the charger will switch to float too early, and it will take a very long time to charge, which could result in sulphated plates, due to never getting to fully charged state, if the battery is too big, then it can be over charged as it becomes fully charged before the charger reduces the voltage, this is more of a problem as the battery gets old, so many chargers have a timer as well, so after 4 or 8 hours (often dip switches to select) even if current has not dropped the charger still drops to float charge.

But the latter stages of charging it is voltage not current that is controlled, current is only monitored it is not controlled, other than a fuse just in case there are shorted cells.

Now with alkaline batteries the reverse is true, current is controlled and voltage monitored, also alkaline should be stored discharged where acid should be stored charged, with so many alkaline batteries we some times forget the lead acid is the reverse in so many features. With a lead acid we can use a hydrometer, with alkaline we do use a hydrometer but not to monitor charge condition, it is only to monitor electrolyte condition and is a completely different hydrometer.

So to repeat the main thing is to check the car or vans voltage, open celled batteries can have a higher float charge as you can top them up, valve regulated batteries need to be monitored more carefully.