Battery Charging

If you need to have the batterys connected in series all the time then its possible to charge just one bank of 12v batterys and have effectively a switch mode charger with 3 isolated outputs charging the other 3 banks of batterys while they remain in series.

You need to consider what the load requirements are quite carefully though as it could get pretty messy if you are drawing a large load off the 48v while trying to charge .. if the load is only on part of the time then a low current charger can do the job and keep the batterys balanced.

paralleling lead acids is used a great deal of the time .. so long as the cell chemistry matches and the temperatures are the same amongst the cells.

I presume you refer to an automobile 12v alternator application.

Question 1 Connecting batteries in parallel will short out the series wired configuration, so the two configurations cannot exist at the same time. except If each battery is individually charged from an independent isolated source. This might be possible with the use of isolating diodes but I have not tried or calculated this option due to each diode's voltage drop which will limit the voltage output and charging ability.

option a - using a set of high current relays, connect up so that the 4 batteries are configured in series whilst you draw power. then energise the relays to reconfigure batteries to parallel for recharging and thus disconnecting the supply to your equipment, if your load characteristics and demand allow, you could rig up a voltage sensor to switch the system configuration into parallel charging if the batteries become discharged down to a predetermined level of +-12.00volts for eg. the alternator should be checked if it can cope with a no load connection while it operates as some alternators will damage their electronics when the load is disconnected, or install a stabilising battery.

option b - duplicate option a, so that when one bank of batteries are charging then the other bank is being used and then after some period of time the reverse is applied. makes for a complicated switching arrangement.

option c - connect the 12volt alternator to charge a single 12volt battery. run a 12VDC to 125vac or 240vac inverter (of sufficient wattage) off this battery, and then use a 125vac or 240vac to 48volt-dc battery charger to charge the 4 batteries that are permanently connected in series, (or just use a 12DC to 48vDC type converter)

option d - If you have enough understanding of electronics then some alternators 12vdc or 24vdc (that do not have a sealed or closed design) can be dismantled and the voltage regulator changed/modified to allow 48volts-dc output instead of 12vdc or 24vdc. When the alternator speeds up at higher rpm, then the ac voltage produced inside the alternator will exceed 50vac. Once rectified; then regulate output to a maximum of +- 58vdc instead of the normal 14.6vdc or 29vdc. This setup would be in addition to and be separate and isolated from the original factory battery and alternator in the vehicle. Granted the full charging amperage may only be available when the engine is revving faster. This is not a faultless option either, but it is an option.

Question 2 - Yeah, the alternator will charge any number of batteries according to the particular internal resistance of each battery. In layman's terms, Internal resistance will vary according to temperature, age of battery, manufacture type, application type, purity of acid, state of charge etc, so no two batteries will have exactly the same internal resistance and hence the same recharge amperage and storage ability at a particular voltage, but by matching battery types and makes etc you can get them pretty close. Charging four batteries that are connected in parallel will just take longer to charge up as they are sharing the available amps from the alternator as you stated.

Years ago I used option-c to run a regular domestic Yamaha amplifier and hifi speakers in my vehicle to get that extra sound quality at high volume when traveling and at parties as I was fed up with the limitations of regular car audio equipment at the time. It was a bit dangerous with high voltage in the vehicle, and who knows if I had been involved in an accident, but when you are younger it doesn't cross your mind.

It appears that you have planned to compose a 48 Volt battery bank, connecting a few units of 12 Volt batteries in parallel to increase the Amp/Hour capacity, and then make three of those groups in series to obtain a total 48 Volt bank.

Alternator will be a 48 Volt charger, to recharge the total bank. On the other hand Recharging any of the batteries individually is not the your target.

Of course this is theoretically possible. 12 Volt auto alternators can produce 48 Volt or some more if they are adequately excited, and in a suitable running speed.

What you only need is to obtain a 48 volt regulator, to fit the excitation characteristics of the alternator in use.

However charging the batteries in parallel groups is not simple as appeared. In fact, paralleling batteries is not a good idea,

For a simple example, if you parallel connect a full charged battery to a full discharged battery, you can not control the high currents to flow in the jumpers from one battery to the other one.

Even you buy all the batteries identical, from the same manufacturers same shelf, they may not having the same specifications by the time. There are many factors to make them different from each other, such as the different liquid level in the batteries, Temperature differences, Acid density differences, the quantity of coagulation at the deep of the battery box etc.

All those differing factors will cause the parallel batteries to get different charges comparing to each other. The charge regulator, at the excitation line of the alternator will not sense the charge of each battery. Therefore you may never be sure all the batteries in your system have completely charged at the end of the charge period.

I advise you that, you may connect the batteries in serial to obtain 48 volt. But try to get rid of parallelling. Try to use a single large capacity of battery of which equal to the paralleled groups. Such as use a single 60 Ah battery instead of paralleling three units of 20 Ah batteries.

Kindest Regards

Thanks for your valued reply.

What I am trying to achieve is to keep a 48v battery bank charged with a 12v 160amp alternator. This alternator produces 160 amps at 750 rpm, which makes it very suitable for a small wind turbine.

The reason that I want to use a 48v battery bank is to match inverters which are easily available and reasonably priced.

I have a 48v deep cycle battery made up of 24 2v cells. If I connected the alternator to every 6 of these cells (4 pairs of wires coming from the alternator), would it work.

Could there be individual voltage cut out switches on each pair of wires to monitor the voltage in each 12v group, or would there be any need.

With a charging source of only 12v, the only way to charge 48v is to step the voltage up to 48volts or split up the batteries into smaller banks of 12v whilst they are charging, which ever is easier for you. I would prefer rectifying and storing 12volts in a battery and then using an inverter to step it up to say 125volts and then using a battery charger down to 48volts for charging the main ban and this way the battery bank stays connected in series.

Modifying the alternator to produce 48 volts dc straight would be simpler, but then you always need high winds to keep the voltage high enough to charge. Alernatively if you store 12volts first then step it up and then down to 48v then wind speed charge threshold kicks in quicker and. I suppose alot depends on the type of wind alternator you have.

See my previous mail #3, the same options and principles apply except you using a different type of alternator Good Luck

As to be an alternative solution, remove the rectifier from the alternator. Instead connect three units of single phase step up transformer, of which approx. step up five times. Those transformers will raise the output voltage of the alternator. Then rectify the raised output voltage for battery charging purposes.

Pls remember the frequency at the output of the alternator will be much higher than the standard mains frequency. Therefore the inductive reactance of the transformers will be higher. This may limit your output Amp. current to flow in the charge circuit.

On the other hand this operation will give you the option of start charging even in lower RPM speeds.

All above operation let's you get rid of rewinding the alternator's stator for the purpose to obtain higher voltage.

By the way, better you get rid of the Solid state voltage regulator, unless the batteries start boiling. It is much practical.

Kindest Regards

simple answer is yes.

but disconect the alternator before you connect batteries in series.

It is possible to remove the rotor coils in a car alternator and replace it with a strong permanent magnet(s). There are companies that do this, or you can do it yourself.

See:-

http://www.youtube.com/

for details.

Remove the regulator and spin it fast, you should get to over 50 volts. Recify with fast diodes and charge your 48 volt battery!!

re replace rotor winding with magnet.

yes you can do this but you would have no electrical control of the output, as out put is controlled by the regulator controling the strenth of the rotors magnetic force.

you would have to control output by speed

it is possible to replace the regulater with a varible output setup say 5volts to 12 volts and set the voltage by varing the rotors voltage this works very well,

I think Andy is right.

Most of the wind power generators are equipped with permanent magnet rotors.

This enable the generator to start charging earlier than the others.

Of course your point, in connection with regulation loss is correct.

Overcharge problem is being solved by switching solid state voltage regulators just after the rectifying unit.

Regards

Your best approach is to,

1/ Find out what rpm your alternator needs to rotate to produce 48 Volts plus the overhead for charging a 48Volt battery bank. Rig your wind generator gearing to produce this but do not expect 160 amp output unless you have a very big wind generator if you get a production of 10 to 15 Amp at 48 to 50 Volt this would be a good result. Also your rectifying diode might need to be changed for the higher voltage rating, check the specifications. You can produce a higher output voltage by feeding the rotor winding with more current for a given speed, but do not exceed the original specification by to much.

2/ remove the 12 volt regulator and replace it with a 48 Volt regulator it might even be possible to convert your 12volt regulator to 48 volt. You should look up some regulator circuits they are not very complicated. It is basically a matter of changing the voltage detection settings and the stabilised voltage level components and possibly the power output control element.

Your 48Volt bank will appreciate this method.

For a off grid house system it is impractical to split the bank for charging, you must charge the whole bank in series. On the assumption that your bank is >600Ah which is likely in this application then you can run your alternator direct to the bank which is then fitted with a dropper regulator which loses any overcharge. This is how all solar regulators work.

Use a 24V alternator from a large truck or coach and bring the stator connections out directly, bypassing the diodes. Rectify it externally. Many diodes that are built into alternators are only rated at about 50V and you will not know whether this is the case or not as the alternator is only rated at 24V.

If you can get the rotor fitted with strong magnets it would be better because otherwise you will have to find a means of switching the field off when the rotor is not turning.

If you want to maintain the electrical rotor field then you will have a problem in that the rotor is usually impedance limited, which allows full system voltage to be fed to the field without it overheating (assuming the unit is rotating correctly and has ventilation). However you will be working at 57V which the rotor was not designed to carry, so some limiting resistor at least will need to be fitted. This will need to drop 28V with about 3A flowing, i.e. 9Ω or 10Ω at 100Watts. As this is not necessarily a precision component you can make one yourself from a ceramic floor tile and ordinary galvanized steel fencing wire. Cut 4mm deep slots in the edge of the tile at about 8mm pitch with a fine grinding disc and wind the wire through the slots so that the wire doesn't touch between turns. You can measure the resistance with the wire straight, before you wind it.

Regards

Chas

I suggest you visit the forum:

www.fieldlines.com

Lots of folks there who have built their own turbines and generators. Many who store using batteries. Lots of good suggestions, solutions, pictures and stories.