When figuring out what wire thickness to use...

No0bert

Member
With sealed lead acid, do you refer to the AH of the batteries to find suitable wire thicknesses, so no extra resistance is obtained. For example, say a 1.5v (AA) battery has .5 Amps, you need a wire that retains .5 Amps, but is it any different with batteries whose SI is Amp-Hours? Just for distinguishing purpose, the batteries are two 12v 18AH sealed lead acid batteries wired in parallel.

EDIT: And if I were to exceed the AWG thickness by a number or two, how bad would this lower the voltage?
 
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BeanieBots

Moderator
No0bert, you are so way off mark with this that I had pause for ten minutes before replying. The very question scares me when I think of how dangerous poor wiring to a Lead Acid battery can be.

Cable thickness and current rating have absolutely nothing to do whatsover in any shape or form with battery capacity.
Cable rating is all based around whatever current your circuit COULD put down the cable under FAULT conditions.

The worst case fault is a dead short. For a lead acid battery, that could easily be several thousand amps. If you look up the required cable rating for several thousand amps it is VERY think. Also, a lead acid battery delivering several thousand amps will quickly boil, explode and spray you with boiling concentated sulphuric acid.

When working with Lead Acid batteries, a FUSE is absolutely MANDATORY.
You MUST have an in-line FUSE.
The rating of the cable MUST be able to carry a higher current rating than the FUSE.

The FUSE rating must be slightly higher than the maximum current that your LOAD is EVER likely to require.

If you have ANY doubts regarding this, for your own sake and that of other around you. DON'T DO IT.
Seek advice and help from somebody who knows what they are doing and can supervise IN PERSON.
 

Andrew Cowan

Senior Member
No.

The AH rating is the capacity of the battery. If your battery has a rating of 18AH, it means that if you draw 18 amps, the battery will last about an hour. It has nothing to do with the current. If you are drawing 0.1mA from your battery, you could use a wire the thickness of a human hair. Low current = low heating, and low current = low resistance. If you are drawing 200A, you will need a big beefy cable (4AWG?)

Once you get below 25AWG or so, the problem is not with resistance. With 1-50A going along a 25AWG wire for a few feet, the voltage loss will be measured in millivolts. However, there will be a heating effect. If you push 50A through a 25AWg wire, alogh it will be fine to start wil, after a few seconds it will get very hot, and possibly melt.

Wire has a rating - eg 100V at 4A. This means that at over 1000V, the insulation can break down. The 4A part means that at more than 4A, the wire will start to get hot.

Batteries all have a maximum current. A (high quality) AA battery's max current may be 1.5A or so - this means that it can never give out more than this, so you will never need particually thick cable. Two 18AH SLA batteries can give 400A+ max, so the thickness of your wire is very important.

To conclude, if the wire is a bit too small, you won't notice a voltage drop, it will just get hot. (When it gets hot, resistance increases, so you may then see a voltage drop).

Hope this helps

A
 

No0bert

Member
So what you guys are saying, is it all depends on how much current I draw from the batteries? I am suspecting around 40 Amps (two 20amps), so my wire thickness will be based off of this correct?

Also, if I wire two batteries in parallel, is it wise to place a 25+amp fuse on the V+ of each battery terminal? (two of them)
 

BeanieBots

Moderator
Yes, put a fuse on each one. That will also help to protect the batteries from each other.

20 amps from an 18Ah battery!
Oh well, at least we won't need to worry for too long and the extra sales will keep battery prices down.
 

kevrus

New Member
Also be aware that Ni-Cd batteries although sometimes small in physical size, have a low internal resistance hence they can deliver quite a high current when shorted out, larger than theirAH rating suggests. As BB says fuses are mandatory and the correct cable size is critical...melted wiring is not a pretty sight.
 

Andrew Cowan

Senior Member
It is not recommended to put SLA batteries in parallel, as when you connect them, they will equal the voltage. If one is charged and the other is not (even by a few volts), the more charged one will attempt to charge the other (at a rate of several tens of amps). Hence boiling sulfuric acid etc. If one battery fails, the other can set fire to it. Never charge in parallel, either.

Also note that the 18AH rating was measured with a discarge of about 1 amp. At 20 amps discharge, expect the batteries to have 70% or so of that capacity (12.6AH).

A

Off the net:

AWG.......Max current (A)
1....................211
2....................181
3....................158
4....................135
5....................118
6....................101
7....................89
8....................73
9....................64
10..................55
11..................47
12..................41
13..................35
14..................32
15..................28
16..................22

So you should use at least 16 AWG to connect the batteries together, and 12AWG for the rest of the wiring. Note - this is if you only draw 20 amps. For the sake of a couple of dollars, you should use wiring with at least twice what you need. If I was doing your project, I would use 9 or 10 AWG wiring for everything.

A
 
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BeanieBots

Moderator
Also, how deep do you intend to discharge them?
What type are they, cyclic or float?
How many cycles do you expect to get from them?
All this affects the amount and rate you can realistically get.
 

inglewoodpete

Senior Member
Also, if I wire two batteries in parallel,
I hope you're joking. Lead Acid batteries have a tendency to explode when used in parallel. Due to their extremely low internal resistance, it one battery develops a fault and its terminal voltage drops a little, the other becomes a current-unlimited battery charger, often resulting in fireworks!

I suggest you do a lot reading on management of lead acid before implementing what you are discussing.
 

westaust55

Moderator
With sealed lead acid, do you refer to the AH of the batteries to find suitable wire thicknesses, so no extra resistance is obtained. For example, say a 1.5v (AA) battery has .5 Amps, you need a wire that retains .5 Amps, but is it any different with batteries whose SI is Amp-Hours? Just for distinguishing purpose, the batteries are two 12v 18AH sealed lead acid batteries wired in parallel.

As a recommendation, alway try and avoid connecting batteries in parallel.

They will have different internal resistance so when charging or discharging, each will accept/delivery a different current. result is that one will be over charged before other is fully charged and one can be dead flat before the other is flat.
 

manuka

Senior Member
Well reasoned responses gentlemen-there's little I can add. Just what however is this 20Amp current flow intended to do? A motor? If so then consider start up & stalling surges as well.
 

BeanieBots

Moderator
Just to re-emphasise. Make sure you fit a fuse on the output of every battery involved with your design.

In all seriousness, please get help from your teacher or parents or some other responsible adult who can supervise your work. Although these are not dangerous voltages, you could easily have a fire and/or boiling sulphuric acid to contend with.
 

westaust55

Moderator
Some battery and wiring fundamentals for you:

1. Batteries are not rated in Amps but in Amp-hours.
2. The Amp-hour rating is usually related to the C/5 current rate for charging and discharging where the number is the number of hours.
3. Batteries can delivery very high currents when there is low resistance (a short)

So if we have an 18 Amp-hour battery then the C/5 rate is 18/5 = 3.6 Amps.

If you increase the current to say 18 Amps, you will not achieve 18 Amp-hours as the higher discharge rate will cause the cell voltage to drop faster and you might only achieve say 30 or 40 minutes so it looks like the battery could only delivery 18 * 0.56 = 10 Amp-hours.

Conversely if the current is 1 Amp, then your will achieve a slightly higher Amp-hour capacity and the battery may last something like 20+ hours.

All battery manufacturers produce curves showing the necessary data including cell voltages relative to current rates and time and often the number of full discharge and charge cycles that the battery will last.
So the requirement here it to read and understand the datsheets.


Fuses do not blow at exactly the fuse rating. The have an inverse curve. So at a few percent overload, they might not blow for some minutes, but at extremely high currents they will blow in a fraction of a second.

The fuses should be rated to match the current of the load. For non inductive loads that will be the next standard size above the load current. For a small motor (say less that 22kW), typically the fuse (or circuit breaker) will be rated at approximately 2 time the motor running current rating. The fuse can still handle the higher starting current for the few seconds involved.

Wiring is usually sized to match the load and fuse rating not the battery rating. The aim is to have a wire size that will carry the current with a maximum desired volt drop. For a motor, that might be say 18 percent nominal volt drop at starting current and 3.5 or 5% volt drop at running current. For short runs usually the current capacity alone determines the wire size but for longer runs, the volt drop will become the deciding citeria.

Others have giving links on data for wiring current ratings (ampacity for those in the US).
Keep in mind though that if the temperature of teh cable is greater than specified in the wiring tables or you have several wires bunched together, they will mutually heat each other if all have current near their capacity and so must be de-rated to a lower current capacity.

The same sized wire will have a lower nominal RMS AC current rating than for DC current. This is due to skin effect where more of the AC current flows near the surface of the wires.


And lastly follow Beaniebots instructions for the location of fuses (at the battery).
 
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manuka

Senior Member
In theory, no. However, there must be some mechanism to prevent this.
Andrew: What DO you mean? I'm only a humble (& late night "Hippy Hours") Kiwi, but since EnZed recently beat Oz. at OD cricket my confidence soars enough to ask for clarification - on behalf of those who may otherwise wilt under the technicalities.

Nor0bert: PLEASE- just WHAT were you intending to do with this 20 Amps of jungle juice?
 
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Dippy

Moderator
I would guess he's planning to power his wheelchair motors?

Andrew, I take it you haven't got a car?
In most cases the battery will just be too flat to crank the engine.
Assuming its not an old kernackered battery or genuinely faulty or it hasn't frozen in Siberia then jump starting is no problem.
And assuming it really is a flatish battery that is the fault.
Step 1. Read the car manufacturer's manual, assuming you didn't buy it from Ebay. If the manual says don't do it for whatever reason, then don't do it.
Step 2. Connect +ve (for -ve ground cars) to 'donor' car battery engine off recommended.
Step 3. Connect -ve. Oooh what a spark!
Start the donor car, then try and start the duff car.

If you had a car and it was stuck then (after sensible checks) the last thing you will worry about is the 0.0001% chnace of explosions. Common sense assumed also ;)
Step 4. Get a membership for AA or RAC or whatever.

And generally, for battery conenctions or anything, plan for the worst case scenario. That should be obvious eh?

You were right BB...
 

westaust55

Moderator
At Dippy's step 3:
Step 3. Connect -ve. Oooh what a spark!​

First connect the ground lead at the "donor" vehicle and when making the connection at the "duff" vehicle, connect to a heavy (eg lifting) bracket on the engine, away from the battery. helpskeep that spark away from any gases near the battery,
 

Dippy

Moderator
Very true.
But when the battery isn't charging then what gases?
When the bonnet is open (and has been open for several minutes to faff around) in the big bad outside world what gases?

More likely petrol / hot oil fumes kicking around, but it never gets a mention.

I agree, safety first, but let's not get too caught up in this.

If it were in an enclosed engine bay like poss in a boat then I'd be a lot more cautious.

Can't beat a drop of common sense.
 

Wrenow

Senior Member
Not top be contrarian, and certainly no particular knowledge on the subject, but.....
We have an engineer who designs battery chargers in our warship combat club - and when queried as to whether to charge in serial or parallel opined that parallel was better (for our SLA's) - yes, he mentioned that fresher batteries would help bring flatter batteries up to the point where all would charge, which slows the charging on the fresher batteries.

I would note that we commonly have two or more batteries (either SLAs or NiMH) in parallel in our ships, and have never heard of a battery exploding from this (yes a good battery will try to charge a low battery and lower your overall run time).

Cheers,

Wreno
 

westaust55

Moderator
People who keep cranking a low battery until it is dead flat may/will create gases at the battery cap vents.

There have been cases where people have caused explosions trying to jump start a car.

Petrol fumes would certainly be another explosion concern. Petrol systems tend to be better sealed in the first instance.

Even Sealed Lead Acid batteries (they are actually valve regulated lead acid) have valves that will release gases if the pressure increase due to excessive charging or rapid discharging. In some "sealed" batteries the regulating values do operate at quite low pressures to release gases.


Safety should always come first.
 

Dippy

Moderator
I know all that Westy....
I've probably jump started a car more times than you have - I had a 10th Hand 1974 Hillman Imp!

So, how much hydrogen hangs around a battery after 10 minutes of the bonnet being lifted?
I realise that hydrogen is a sticky and very heavy gas, but, in the Real World, after several minutes of the driver giving up, 20 minutes to get their mate out of the pub and opening the bonnet to faff with cables ... then... oh really...

There have been incidents of everything mate ;)
One incident in 1000000000 jump starts I guess - and we don't know the true circumstances.
I'm not telling people to be careless, just use common sense.
Anyway, enough of this (b)anality :)
 

westaust55

Moderator
I know all that Westy....
I've probably jump started a car more times than you have - I had a 10th Hand 1974 Hillman Imp!

So, how much hydrogen hangs around a battery after 10 minutes of the bonnet being lifted?
I realise that hydrogen is a sticky and very heavy gas, but, in the Real World, after several minutes of the driver giving up, 20 minutes to get their mate out of the pub and opening the bonnet to faff with cables ... then... oh really...

There have been incidents of everything mate ;)
One incident in 1000000000 jump starts I guess - and we don't know the true circumstances.
I'm not telling people to be careless, just use common sense.
Anyway, enough of this (b)anality :)

so my x-hand 1958 Hillman Super Minx does not count :(
 
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papaof2

Senior Member
I did see a battery explode once. The car was driven onto the service lift, then raised, the starter removed, then the car lowered to have access to the battery. I don't know the total time, but probably at least 20 minutes. The mechanic placed the starter terminals onto the battery terminals and the side/bottom of the battery exploded out. Fortunately, the force of the explosion was all either downward or toward the engine so there were no injuries (except to the shoes I was wearing).

Ive jump started vehicles for more than 30 years with no incidents, but I always make the last connection to the engine block, not the battery.

John
 

krypton_john

Senior Member
I hope you're joking. Lead Acid batteries have a tendency to explode when used in parallel. Due to their extremely low internal resistance, it one battery develops a fault and its terminal voltage drops a little, the other becomes a current-unlimited battery charger, often resulting in fireworks!

I suggest you do a lot reading on management of lead acid before implementing what you are discussing.
Guys, don't large boats, electric golf carts etc always have banks of parallel connected lead acid batteries?
 

BeanieBots

Moderator
Oh well, as we're all discussing it...
Batteries even have the ability to explode all on their own whilst sitting in the corner not connected to anything.
Oxides form which fall from the plates, these can build up at the base of the battery and short the bottom of the plates. This then causes the plates to buckle which then makes for further shorting resulting in a run-away.
Fortunatley, this only happens in poor condition batteries and the short current is often not very high.

How rare? I know of two incidents. Both fairly harmless but did melt the sides and make a mess.
 

Andrew Cowan

Senior Member
Guys, don't large boats, electric golf carts etc always have banks of parallel connected lead acid batteries?
No. They are connected in series. Higher voltage = lower current (for a given wattage), thus less heat. The old P=IIR.

That's why milk floats are 96V - low current, low heat, easy to control.

A
 

westaust55

Moderator
Guys, don't large boats, electric golf carts etc always have banks of parallel connected lead acid batteries?
I guess in part it can depend upon the quality of the electrics (and not neccessarily the price).

There are plenty of wheelchair and even electric bicycle motors that run from 24V and 36V so the batteries can be in series rather than parallel.

Like papaof2, I have jumped a "few" cars in my time without a mishap but have personally seen a battery explode in a car only 3 metres from me.

How many cars have starting battereis in parallel?
They size one battery to do the function.
Heavier, non air-start, machinery including large trucks and bulldozers have batteries in series, never in parallel.
 

papaof2

Senior Member
parallel batteries

How many cars have starting battereis in parallel?
Some of the US passenger vehicles and light trucks with diesel engines have two 12 volt batteries in parallel to provide the extra power needed to turn the engine against the higher compression. A 24 volt system would be better, but would lead to total confusion if one Ford F350 truck had a 12 volt electrical system and the next one used 24 volts. How long does it take to burn out a 12 volt headlight ($10) or engine control unit ($300) with 24 volts applied?

Some people just don't understand the differences between gasoline and diesel engines. I overheard the following exchange at an auto repair shop (Sears) in the days of passenger cars with GM's diesel 350 V8 (a piece of incredibly bad conversion engineering - GM later replaced all those diesels with the gasoline 350 V8 at no charge).

Potential Customer: "None of my other cars have two batteries. What happens if I only install one battery instead of two?"

Manager: "The car probably won't start in cold weather and you will void the warranty on the one battery you install."

Potential Customer: "I won't buy two batteries."

I don't know the final outcome, but the potential customer's tone of voice had reached the "You're ripping me off!" level.

My neighbor with a diesel Ford pickup dutifully plugs in the engine heater each night if the temperature is expected to drop below 40F or so - anything to make cold starts easier.

Maybe I could design a PICAXE-based rate-of-temperature-drop device to tell him how early in the evening he should plug it in?

John
 

BeanieBots

Moderator
It's a question of correctly sizing the battery. With Pb, there is no excuse.

LiPo's are commonly done both ways. The battery includes a descriptor "XSYP" where the X is the number in series and the Y is the number in parallel.
Hence 3S2P would be 3 cells in series paralleled with an identical set of 3.
For example, this one.
http://www.maxamps.com/Lipo-8000-111-Pack.htm

It should be noted, these are MANUFACTURED that way. Hence no possibility of mixed charged with discharged or old with new. Nonetheless, self discharge will always be higher than non paralleled packs.
 

Wrenow

Senior Member
Guys, don't large boats, electric golf carts etc always have banks of parallel connected lead acid batteries?
Depends. I have seen golf carts running 36v with 6 6v in series, but I seem top recall seeing some with 6 12v, in two parallel sets of 3 in series. Screw up the wiring and you get some nice spitzen sparken.:)

Anecdotally, my brother's boat has two that are charged in parallel through an isolator (a starting battery and an accessories battery). When queried, he indicated many do not bother with the isolator, some have a L/R/both switch, etc., Several ways to witr\re them up, depending on what you want to do (my brother wants to be able to drive his accessories late into the night until the battery may be low and still be able to start his boat with a fresh battery).

My 1958 Jag (not to be excited, it was a beater, but I had a ton of fun with it) had twin 6v batteries in serial, one behind each front wheel.

Cheers,

Wreno
 

inglewoodpete

Senior Member
Jump starting car is usually OK due to the (relatively) high resistance of the jumper leads or connections (where the sparks happen :)) This limits current flowing from the fully charged battery to the flat one. Often you only need a few 10s of amps from the added battery to get the car started.

The problems come when you put good, fat, low resistance wiring between two paralleled batteries. At best, they will self discharge to the voltage of the poorest battery. If the batteries are not perfectly matched and remain that way through their entire life they will not give their best. If they are charged independently and then connected together with those fat cables I mentioned earlier, then expect fireworks.

It's like trying to hit a nail with an undersized hammer. Don't use two hammers, get a big one!
 

manuka

Senior Member
It's like trying to hit a nail with an undersized hammer. Don't use two hammers, get a big one!
Or hit the nail twice with a lighter (but more convenient) hammer?!

FWIW check the 2007 account =>http://www.automotivedesignline.com/howto/199201615 regarding the move to "42V" car batteries. This higher voltage means lower currents (allowing thus cheaper & lighter copper wire & more compact/lighter batteries) for the same power. They're nominally 3 x 12V of course, but quoted as 14 x 3= 42 V as this will be the fully charged EMF. I've been keeping an eye out for them, but as yet they're elusive. Given the hybrid rise it's possble they'll now merge into an e-car propulsion system- some of these are 100s of Volts.
 

BeanieBots

Moderator
Hybrid batteries (mine included) are typically 300-350v.
Don't forget, they need to deliver peek power in the region of 50kW:eek:
That would be 1200A even from a "42v" battery.
 

westaust55

Moderator
Batteries and wiring sizes

So by now, No0bert should have all the warnings and basic information to study up further and make sure the project is done right . . . . ;)
 
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