Wire size to charging battery?

[TheRealAndy]

I forgot to say to the driveon's solutions is a great one too, and I have seen this used before. The more current you draw through a lighbulb, the hotter it gets and the higher the resistance. These make great simple regulators. Any bulb will do, but a headlight bulb will allow higer charge current, as opposed to an indicator bulb which will be a lower charge current. One could even use a 240 incandecsent bulb, however these are a bit fragile.

[driveon]

Hi Andy, if your coming off an alternator then you can use just about any size globe.

If you are powering the globe from a power supply that is higher than the maximum tolerable voltage of the battery then it is safer to use the smaller globes as suggested, the smaller globes not only limit the charge current but once the battery is fully charged, the internal resistance of the battery will be sufficient to counter the current coming from the power source and the battery will simply remain fully charged but if you use a larger globe then the current coming through globe will be too high for the battery to take.

So while you use a small globe, you can leave the battery connected to the power supply all the time with no risk of cooking the battery but if you use a large globe, you will either have to remove the charge supply once the battery is fully charged or risk cooking the battery if you don’t remove the charge supply.

Cheers.

[davez104]

Wouldn't the power from the car alternator be regulated anyway? I must be missing something because I can't see how you would cook a 12V battery by hooking into your car power supply.

Dave.

[TheRealAndy]

Wouldn't the power from the car alternator be regulated anyway? I must be missing something because I can't see how you would cook a 12V battery by hooking into your car power supply.

Dave.

Its regulated but can supply a lot of amps. Most cars i think have around 75A altenators. IF you let a small battery draw 75amps it would probably explode! SO the trick is to regulate, or limit the current that the smaller battery can draw when charging

[oldboot]

I have edited my original post here because a lot of the information was either irrelivent or wrong.

cheers

[oldboot]

yep I remover this one too.
cheers

[oddbudman]

One point I need to make seperately.

There is no danger of damage to a sealed lead acid battery from being short term charged at 15 volts.............I have done it many times.............many of the clever chargers do it in boost charge mode.

cheers

Yes they can do it in boost mode, but they do set a current limit into the battery.

You just stated ohms law, so if we follow this connecting a 11V battery to a 15V source with basically a short circuit (the hookup wire) is likely to cause a big current spike into the 11V battery. Get a meter and test it. A small 2.5Ah battery is likely to have a inital input current specification of around 0.3C or in this case 0.75A, connecting a empty battery to 15V will get the charge up, but it will damage the battery if you do it.

A series resistor will help to reduce this input current rush, as it can bias the current that is fed into the battery.

Your point on excessive charge voltage into a battery causing overcharging is 100% correct.

oddbudman

[finga]

I think you fellas have got way over the top of the purpose of the thread.

Bungie just wants to charge his SLA battery from the car.

He wants to put and auxiliary fuse panel in (and yes 4mm wire is dokky dokky but use a fuse to protect the wire from over current situations from the main battery to this extra fuse panel).

Charging of the SLA is easy then.
Just bung one of these in.
http://www.jaycar.com.au/productView...Max=&SUBCATID=
A couple of wires to the axillary fuse panel (so it's protected with a fuse and you could even put a switch in if you want to leave it in the ute permanently) and then a couple of wires on the output so you can attach it the your little bubba SLA battery.
Jobs done. All automatic, cost effective, no ifs and no buts about it.

[oddbudman]

Finga,

The input voltage on that unit is 16-20VDC (solar panel).

The jaycar blurb is a little misleading, it should probably say... "a rectified and filtered ac input of 13.8-15VAC".

http://www.kemo-electronic.com/pdf/m083/m083.pdf

oddbudman

[TheRealAndy]

Actually, for cyclic use 15v (14.9 actuall) is fine according to this reputable manufacturer:http://www.panasonic.com/industrial/...LC-R122R2P.pdf

I assume based on the description from bungie that the battery is going to be used for cyclic use.

So the problem then comes down to current limiting. Short of buying some expensive 12v - 12v charger, then i think the light bulb has merits. Sure it wont prevent overcharging, but small SLA'a are cheap and thats the tradeoff between spending hundreds on a proper charger as oppose to throwing away a $20 battery once in a while.

Using the lightbulb and looking at panasonics website, dont leave the battery on for more than 12hours and you will be fine

[oldboot]

this one too.

cheers

[oldboot]

Due to the great amount of " strong opinions" including my own, I thought it would be "interesting" to do some actual testing.

And the results were surprising......... in short none of us " experts" got it all right.


So
I statred up my old faithfull corona fishing waggon and ran the motor to let it charge the battery for an hour or so......I checked the battery terminal voltage.
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13.78 Volts ........ I was firmly convinced it would be more......... so I give the motor a good long rev........ nup doesn't move.

So to confirm that the alternator has finished charging & itsn't playing a trick I put my high res DC clamp meter on the alternator lead....5 & bit amps..... consistent with an ignition load....... clamp the battery lead..... diddly squat.

HMMMMMM

I take a well used 12V 7Ah gell cell and flatten it proper with a 50 watt dichroic lamp & let it cool....... resting terminal voltage under 8 volts way below recommended...... the halogen light was well yellow.

I hook this directly to the battery terminals and measure the current with the clamp as it is connected and observe.
initial current just over 7 amps and it reduces rapidly to about 4 amps in under a minute... to drops to under 5 within about 20 or 30 sec.

As ve speak it is charging I expect it to be fully charged in about 2 hours, give or take.

This is all looking a lot simpler than we all thaught, certaily much safer and decent than I expected.

Further testing will insue later today.....I have a near new 1.2Ah 12V cell that I have flattened.

will report later.

So this all looks very viable.......I have hesitated to recomend or use this method myself for many years because of the "accepted problems".

I've been arround car electrica and this sort of stuff but never realy bothered to measure running terminal voltages....... or more likely believed what I have been taught and read & not what I measure.

To a certain extent what I am measuring actualy makes more sence..... or we'dd be stewing battereies left right & centre

I would be most gratefull if various member that have the instruments could measure the terminal voltage of their car & boat battery with the engine running after a good long run.....

I want to establish what is correct and normal.......! decimal place ... or better two.

I now know what I expect to see......

. the alternator on the "rona" is a reasonably bew bosch with inbuilt reg...which is pretty common.

cheers

[oldboot]

All right testing completed, I find my self red faced, I'm happy to say when I am wrong.

so I found

Yes these SLA battereies will draw excessive current when charged directy from a car charging system.

There seems to be quite a bit of variability between different batteries, and the smaller the battery the bigger the problems.

I tested a couple of cells arround the 7 & 8 AH & they seemed to stablaise at arround half their rated capacity in charge rate....... which is still too high....1/3dr is the recommended maximum rate.
This size of battery warmed up, more than I would like...this wouldnt be wise long term.

The 1.2 AH battery I had on hand drew quite high currents to start with and stabalised at arround 2.5 amps..... which is way too high......and because of the small size the battery heated up very quickly causing me to treminate the test.

So my conclusion on direct connection to charge.......batteries larger than 7Ah you will probably get away with it. but it wont be good for the battery......I would consider it only a viable emergency measure.

Anything smaller.....I would strongly recomend against it.......little 1.2 or 2.5 Ah cells are likely to get very hot and vent acid or worse...... don't do it.

I recon once you got over 20 AH you would be pretty safe....... but I have no proof.

Now the diode thing is totaly useless, because if all the alternators are as I have found & I dont see why they wont be....... terminal voltage isnt a problem.

I tested both the resistor and light bulb options....

I tried 5 watt and 21 watt light bulbs......both reduced the current but neither allowes sufficient current to achieve a rapid enough charge and insufficient current was passed to allow the light bulb to do its "thremal dynamic thing"

The light bulb will work but It isnt all that convienient and you would have to try several light bulbs to get an effective result, and no dynamic action is achieved the light bulb is simply acting as a resistor.

The resistor has my vote, a 3.9ohm resistor reduced the current to arround 300mA which is border line acceptable for the 1.2 ah cell.
The same resistor was tested with a variety of batteries & seemed to result in similar charge currents on all cells.

The problem is the resistor will need to be selected for each different capacity cell, & I would argue by trial and error measuring the charge currents.
A small value resistor say under 4 ohms is unlikely to have any significant effect on end of charge terminal voltage as the current flowing should be very low at this point.

As recomended earlier by someone else a 3.3ohm 5 watt resistor is as good as it will get for a 2.5AH gell cell.........the resistor tested didn't seem to get hot at all.... but one should assume that it will for safety sake..

I intend to do more testing ith different batteries as getting this all down would be very usefull to a lot of people , myself included.

I appoligise for my earlier incorrect asumptions and misleading recomendations.


As to the wire size...... for charging anything up to 10 AH 1.9mm2 wire should be fine, I think that equates to 4mm auto electrical wire. as long as the length was less than say 3 metres.

cheers chaps

[TheRealAndy]

If there is enough interest I will design a small charging using a topology that makes input voltage irrelevant. I am not confident enough in my abilites to do something for larger batteries, but small batteries less than 5AH or so is no problem. You can probably do it all using one IC these days . I have had intentions to do this for sometime now, but I have been to lazy.

[oldboot]

One of the electronics magasines published a dc to dc converter based unit specificlay designed to charge sealed lead acid batteries at one stage.
Jaycar used to stock it.....I haven't looked for it in the catalogue recently.

For small to medium batteries the series resistor isn't wonderfull but it is cheap and works.

The light bulb also works but by the time you buy a big enough light bulb to be effective it starts to look expensive and cumbersome.

Cheers