Lead Acid Battery Charging - the principles? - Motorhome Forums, Motorhome Discussion, Motorhome Chat

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post #1 of 20 (permalink) Old 02-11-2012, 04:14 PM Thread Starter
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Lead Acid Battery Charging - the principles?

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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post #2 of 20 (permalink) Old 02-11-2012, 04:36 PM
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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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post #3 of 20 (permalink) Old 02-11-2012, 04:41 PM
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"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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post #4 of 20 (permalink) Old 02-11-2012, 10:19 PM
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Quote:
Originally Posted by DABurleigh
"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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post #5 of 20 (permalink) Old 02-11-2012, 11:02 PM
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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.

Frank
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post #6 of 20 (permalink) Old 02-11-2012, 11:19 PM
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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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post #7 of 20 (permalink) Old 02-11-2012, 11:21 PM
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Quote:
Originally Posted by bigtwin
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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post #8 of 20 (permalink) Old 02-11-2012, 11:47 PM
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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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post #9 of 20 (permalink) Old 02-11-2012, 11:51 PM
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Quote:
Originally Posted by H1-GBV
Quote:
Originally Posted by bigtwin
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
NOTHING happens "becuase" of a law. The law describes the relationship
My point exactly. We concur!
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post #10 of 20 (permalink) Old 03-11-2012, 12:52 PM
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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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