# Lead Acid Battery Charging - the principles?



## jonandjules (Jun 21, 2009)

Question? When a 12v lead acid battery is being charged, is the current 'forced' into the battery or does the battery just 'take' the current it needs?

My motorhome has an 85Ah starter battery and a 220Ah leisure battery and when driving or stationary and plugged in to 230v the batteries are charged in parallel (I have tested this with a meter).

As there is a large difference in size between the two batteries surely one of them is fully charged before the other?

My motorhome charging system has a 12v alternator on one end and a Waeco 3 stage charger on the 230v side. Both the 12v and 230v systems charge both batteries. Between the 12 v and the 230 v charging systems is a CALIRA TREMAT TS70 battery switching relay - does this know when each battery is charged?


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## listerdiesel (Aug 3, 2012)

The battery voltage is raised to an artificially high level by the charger, the battery normally sits well below the charging voltage when off charge and off load.

If the two batteries are solidly in parallel, then the one with the lowest state of charge will determine the overall charge progress.

Peter


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## DABurleigh (May 9, 2005)

"Forced versus take?" - the same, it's just Ohm's Law.

"As there is a large difference in size between the two batteries surely one of them is fully charged before the other?"

Well, with both batteries at the same partial state of charge, more current will go into the vehicle battery simply because there is less cabling resistance as it is closer to the alternator (Ohm's Law again). This is GOOD, because otherwise you would be charging mismatched batteries equally, which would knacker both quite quickly.

I suspect your TS70 is a modern equivalent of the split charge relay that disconnects the batteries when the alternator isn't charging. My mains charger charges both batteries but the vehicle one is connected to a separate low-current (4A max) output. If my mains charger were in a "battery-safe" charging mode that limits its voltage, I wouldn't object if the batteries were connected and charged on the same output. But if my mains charger were in an aggressive (maximum battery capacity) charging mode, I would NOT want my vehicle battery connected in parallel, despite the resistance in the cables.

Dave


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## bigtwin (May 24, 2008)

DABurleigh said:


> "Forced versus take?" - the same, it's just Ohm's Law.


Your answer may be correct but it is not, I believe, because of Ohm's Law. Ohms law merely defines the relationship between voltage, current and resistance. Thus using Ohms law it is possible to determine how much current will flow. It provides no means to explain how the current flows.

It is my understanding that the direction of flow (current or fluid) is determined by the 'potential difference' between two points. The 'medium' will always seek the path of least resistance and will flow until there is no longer a potential difference.

Thus, the contents of a gas bottle (high pressure inside) will flow through a leak path (this is effectively the resistance in the circuit) to the the lower pressure side (atmosphere). Flow stops when the potential difference between the inside and the outside is zero, i.e. when the pressure is the same. This explains why there is a flow but not whether the gas is pushed out or pulled out.

Consideration of the 'lift' forces acting on an aircraft may be analogous? The aircraft is not held aloft by air pushing up under the wings but rather by the low pressure zone above the wings (creating the potential difference) 'sucking' the wing upwards.

This still hasn't answered the OP's question!!!

I suspect that the real answer may require an understanding of the physics/chemistry of the issue. This' I'm afraid, is an understanding I don't possess!

Ian


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## bigfrank3 (Mar 26, 2007)

IMHO It's Kirchoffs law which you need but you also need the understanding as Kirchoff states in his first law that the algibraic sum of currents in any junction equate to zero. And further the sum of currents into a junction equal the sum of currents out of a junction :? 


The battery will only take what it requires as it's internal resistance will rise as it becomes more charged and thus limit the current.

As I have always known, a little bit of knowledge can be dangerous and I always say there are different levels for diferent people.

I'm sure someone with better knowledge than me will be along to straighten us all out. :lol: 

Frank


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## CliveMott (Mar 10, 2008)

Its not that difficult.

A few basics

At any point in its charge or discharge cycle the actual internal voltage of the battery will be of a specific value due to the chemistry.
At any point in its charge or discharge cycle the battery will have a specific internal resistance.

Simliarly the charger will have an internal resistance as well as an internal voltage at any specific moment. These can change as most chargers today are controlled electronically so both the chargers momentary internal resistance and voltage are changing.

All the wiring connecting these together plus any fuses will also have a resistance value.

So for example lets say during charging the battery internal voltage is X and the charger internal voltage is X+1 then the current from the charger will be the difference in these voltages (1) divided by all the internal resistances and external resistances added up.

When two (or more) batteries are connected in parallel the individual internal voltages and resistances can be different and this will allow some current to flow from the battery with the higher internal voltage into the battery with the lower internal voltage. But if on charge the charger voltage will be even greater so the battery with the lower resistance and voltage will have a larger proportion of the charging current.

Do the analogy with water butts and hose pipes if you like and its just the same.

C.

What was the question?


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## H1-GBV (Feb 28, 2006)

bigtwin said:


> Your answer may be correct but it is not, I believe, because of Ohm's Law. Ohms law merely defines the relationship between voltage, current and resistance.
> Ian


An interesting pair of sentences illustrating how we use language loosely at times.

NOTHING happens "becuase" of a law. The law describes the relationship (and it often has provisos which we ignore for brevity).

The laws which most appropriately describe currents and voltages in these situations are probably Kirchoff's Laws. (Just spotted Frank's post of 10.02pm)

Analogies to electric current may also not be helpful: in a series circuit the instantaneous current is the same at every point (ignoring relativistic effects, etc). The current leaving the positive pole of the alternator is the same as the current entering the negative pole, so there is no meaningful concept of "pulling" or "pushing".

In basic terms, the power supplies convert their input power (motion, electricity) into output power (electricity) and a current flows which depends upon the net voltage and the resistance. As the battery charges, the net voltage falls and so does the current. (Devices which can output a "constant" current give a different perspective). This current causes a reversible chemical change in the materials inside the battery: when the battery provides the power, the chemical change occurs in the "opposite" direction. (As the chemical change occurs, the total resistance of the circuit changes, so the process is even harder to describe succinctly.)

If you find it confusing, just think of it as "magic" and be grateful that it happens. When it stops, buy a new battery or, less frequently, a new charger.

Enjoy a useful energy source - Gordon


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## bigtwin (May 24, 2008)

A great few posts that do provide further insight but, like mine, none of them add further clarity to the OP's question!


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## bigtwin (May 24, 2008)

H1-GBV said:


> bigtwin said:
> 
> 
> > Your answer may be correct but it is not, I believe, because of Ohm's Law. Ohms law merely defines the relationship between voltage, current and resistance.
> ...


My point exactly. We concur!


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## DABurleigh (May 9, 2005)

Well, fortunately, as Clive will testify, I DO understand the Physics, but the point you all seem to be missing is the CONTEXT of the OP's question, which at least I tried to address! 

I derived Maxwell's equations from first principles, added in Einstein's relativity, which then explains the whole of electricity, magnetism and electromagnetism in a single simple, very concise, equation, but it won't help the OP one jot!

Dave


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## safariboy (May 1, 2005)

Ohm's law -often quoted in this sort of context- really only states that the resistance is constant. It may or may not be true. Lamp bulbs, a gas, salt solution, and diodes are all examples of things that simply do not follow Ohm's law.

As for the plane wings the basic physics is very simple even if the practice and efforts to explain it can make it as clear as mud:
The plane pushes air down: It follows that the air pushes the plane up. this is Newton's third law. The complicated shapes of a wing are designed to do this with as little drag as possible.


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## DABurleigh (May 9, 2005)

No-one said resistance is constant. The internal resistance of a battery changes throughout the charge-discharge cycle, which is PRECISELY the reason why one is recommended to replace parallel charged batteries with identical ones.

Dave


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## jonandjules (Jun 21, 2009)

Well chaps, thank you so much for all the replies.

So, is it OK to let the 230v charger charge the starter battery in parallel with the leisure battery?

The Calira Tremat TS70 that 'sits' between the two batteries is a type of split charge relay. I have attached a pdf. My motorhome is wired exactly as is shown in the document.


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## CliveMott (Mar 10, 2008)

"Well, fortunately, as Clive will testify, I DO understand the Physics,"

Well I agree that Dave did understand all this once when he was younger. Today I am not so sure!!!

C.


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## Bazbro (May 9, 2005)

I was confuserised :? until I read *H1-GBV*'s explanation...

_"If you find it confusing, just think of it as "magic" and be grateful that it happens."_

Thank you Gordon. Now I understand and I'm happy!


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## peedee (May 10, 2005)

Cannot it not be explained more simply.If they were simply in parallel with no intelligent charger like the Tremat the lower capacity battery would charge first but the charger voltage would still remain high because the larger battery isn't fully charged. You would then be overcharging the fully charged lower capacity battery which is not going to be very good for it. The Tremat would appear to disconnect the engine battery once it is fully charged therebye insuring it is not damaged due to overchaging
peedee


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## jonandjules (Jun 21, 2009)

peedee said:


> Tremat would appear to disconnect the engine battery once it is fully charged therebye insuring it is not damaged due to overchaging
> peedee


That's what I was hpoing to hear!!

When I read the instructions I am not 100% sure that that is what happens.

I have let the charger charge both batteries a couple of times and checked the voltages with a meter but each time I get both batteries reading the same 14.+ volts. (charger is set at 14.4v max).

At the moment I have the fuse removed so the charger is only charging the leisure battery.


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## IanA (Oct 30, 2010)

safariboy said:


> As for the plane wings the basic physics is very simple even if the practice and efforts to explain it can make it as clear as mud:
> The plane pushes air down: It follows that the air pushes the plane up. this is Newton's third law. The complicated shapes of a wing are designed to do this with as little drag as possible.


Now we are getting into Bernoulli's Theorem and the relationship between gas flows. A wing 'lifts' because the distance the gas (air) has to pass over the wing is longer than the distance under the wing. The speed of the gas passing over the wing is therefore higher, so the pressure is lower - the wing is 'pushed' up by the higher pressure beneath it. This is all linked back to the Gas Laws - Boyle et al.


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## nicholsong (May 26, 2009)

IanA said:


> safariboy said:
> 
> 
> > As for the plane wings the basic physics is very simple even if the practice and efforts to explain it can make it as clear as mud:
> ...


Ian

That is the oft-quoted explanation, but it is more complicated than that. Safariboy was right in saying that air is pushed down - but did not elaborate. The air passing over the upper surface of the aerofoil when reaching the trailing edge does indeed 'push downwards', but is also draws air from above the wing to create a downward force greater than would be explained by Bernoulli's Theorem/Law. This downward force brings into play Newton's Law to produce an opposite upward reaction.

Even that explanation is a simplification of what is happening and a full explanation will follow when I have completed my degree in aerodynamics 

Safariboy was correct in what he said but maybe a bit too 'succint - fair enough since we are 'off topic'' :wink:

Geoff


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## airstream (Jan 7, 2009)

*Back to batteries*

Getting back to batteries

Even the cheapest canal boat engines have two alternators fitted one for the engine start and one or more for the domestic batteries

All canal boat engines are derived from auto engines - mine is an Isuzu engine with 3 alternators - none turbo version of a Trooper

All the charging problems associated with charging multiple battery banks from one alternator could be solved at low cost if a "motorhome" specific chassis was fitted with a second alternator

Just food for thought and a DIY job for those who have the room to fit another alternator

Ray


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