Assault and battery

[neptune]

Maybe that should have read assault on battery. It puzzles me a bit that on an Ebike with a 36 volt SLA battery we can draw 5 or 6 amps , with peaks of 20 amps or even higher. And yet when it comes to charging , my charger outputs 1.8 amps. As far as I know, the batteries do not overheat on discharge. So why can we not charge them at 5 or 6 amps? Will they overheat, or have a smaller number of charge discharge cycles? Or is it just that chargers with the lower output are cheaper to make. The obvious advantage of a higher charge rate would be obvious if you stopped for a recharge on a long journey. Any theories?
Also it always amused me when people took a battery off a vehicle to charge it with a 4 amp battery charger, and removed all the cell tops. Then they put it back on the vehicle, with the cell tops on, and drove off with the alternator charging it at 30 amps or more.

 

[rog_london]

I would surmise that not only are the lower-current chargers cheaper to make, but also easier to control. Usually the main problem is with serious overcharging, and it's much easier for the BMS or the charger itself to keep track of a relatively low current.

Back in the days of NuCds, some were made which could happily accept a very high charging current - so a full recharge in less than ten minutes from flat was possible. However, there were serious problems with this, not only in detecting the right moment to switch the charger off, but also with the real risk of an explosion due to heat build up if the charger kept going. Also some cells would inevitably have a higher capacity than others, and the ones which 'got full' first would get hammered and soon deteriorate further.

In the case of vehicle batteries, the reason for venting the cells was that chargers often just comprised a transformer and rectifier - so the battery would get overcharged readily if the charger were left connected. This wasn't a serious problem with old fashioned lead acid because all that happened was that the water in the battery would get dissociated into hydrogen and oxygen - and venting the cells prevented a pressure build up. As long as you didn't charge it until there was a serious loss of liquid (i.e. you kept it topped up) no problem.

Sealed lead acid batteries are able to recombine the hydrogen and oxygen as fast as it's produced - as long as you don't overdo it. You generate a bit of heat instead. However, if you use a 'dumb' charger it must not have too high a rating.

A smart SLA charger can hit a flat battery (even an SLA) pretty hard - I've got one which does 30 amps into a flat vehicle battery. No problem - as the battery gets charged its terminal voltage rises and the charger reduces its output accordingly, finally switching to a fixed voltage (about 13.5V) which the battery is happy to 'float' at.

An alternator does something similar - it has a voltage regulator built in which maintains the battery plus all the load you can give it. It only hits the battery hard if it's obviously flat. Therefore, no venting needed and no topping up. It also produces useful output at low revs, i.e. tickover.

Back in ye olde days when you had a dynamo, electromagnetic cutout and 'regulator', you would overdrive the battery when you were travelling at speed with no accessories turned on, but you would get no help from the dynamo at tickover. So, topups with distilled water were a normal fact of life, and in winter you turned your headlights off if you were in a traffic queue or you risked a flat battery.

Rog.

 

[oigoi]

As I understand it with lead acid batteries when charging them it takes time for the charge to soak into the plates of the battery, so charging them at a really fast rate the battery would appear (from measuring the voltage) to be full when it could really store more charge.

therefore charging them at a slower rate for a longer period of time makes sure the battery is storing as much charge as possible

 

[neptune]

Thanks for those interesting replies. Back in the 90s I used to build my own chargers of the simple transformer rectifier type . To some extent these chargers are self regulating, as the battery voltage rises , it increasingly opposes the incoming current, reducing the charge rate.The trick is winding the transformer to give just the right voltage. The reason why an alternator will charge at tickover and a dynamo will not, is not an obvious one. Generally speaking, an alternator has a smaller pulley than a dynamo, so at a given engine RPM. it runs faster than a dynamo would. You could not do this with a dynamo, because the more complex rotor [armature would not hold together at peak engine RPM. The commutator would open up like a bunch of bananas!

 

[flash]

Thanks for those interesting replies. Back in the 90s I used to build my own chargers of the simple transformer rectifier type . To some extent these chargers are self regulating, as the battery voltage rises , it increasingly opposes the incoming current, reducing the charge rate.The trick is winding the transformer to give just the right voltage. The reason why an alternator will charge at tickover and a dynamo will not, is not an obvious one. Generally speaking, an alternator has a smaller pulley than a dynamo, so at a given engine RPM. it runs faster than a dynamo would. You could not do this with a dynamo, because the more complex rotor [armature would not hold together at peak engine RPM. The commutator would open up like a bunch of bananas!

Back in the day when I was a newly qualified auto electrician and the first Lucas ac10 and ac11 alternators were been fitted, myself and a colleague had this same discussion. He had a Standard 10 with a triumph herald engine fitted. He decided to to turn down the pulley of his dynamo on a lathe to make it spin faster at lower revs, with just the results neptune predicted the commutator flew apart. Didn't notice any bananas though

 

[flecc]

On fast charging lead-acid, back in the 1950s there was a sudden craze for fully charging car batteries in under 30 minutes from flat, fast charging agencies popping up all over the country. Basically the battery was placed a a tank and packed around with dry ice to counter the high heat generation, then blasted at the very high charge rate necessary. It all disappeared very quickly, possibly due to accidents!

 

[neptune]

My experience of vehicle alternators and dynamos dates back to the 1970s. Together with a group of hippy friends, I was involved in a project to design and build small wind generators. There is a limit to the shaft speed you can achieve with a small wind rotor. Bigger rotors are more powerful, but slower. Gearing is lossy and expensive. We decided a good compromise was 6 feet diameter. At useful wind speeds, this will rev at about 200 to 600 RPM. Car dynamos and alternators need a minimum of 1200 RPM. We rewound a dynamo armature with more turns, using thinner wires . It took us weeks, and several attempts. Results were disappointing. The best we got was about 2 amps in a 20 MPH wind. Alternators were a bit better, and were easier to rewind. We could get about 5 amps @ 11 MPH. We worked out that in standard configuration, a car alternator is 27% efficient!
One significant reason for the poor efficiency is that you need at least 2 amps to excite the rotor [field].With the availability today of high tech permanent magnets, you can build a generator that is 80% efficient or more. Still we had lots of fun, and learned a lot, and it kept us off the streets. One result of this work, was that I learned how to rewind transformers, which was useful in building battery chargers.