Voltage drop on DC system

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Old 05-08-14, 03:53 AM
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Voltage drop on DC system

To give you some background to my next question I have 3 60 AH deep cycle batteries giving me a total 180 AH. They are charged via solar panels and routed via charge controller.

My question revolves around voltage drop and how to eliminate it or what can I do about it rather.

All values stated here are readings obtained while the system is under load ie lighting the yard.

I am a bit perplexed, if I take a voltage reading at the batteries, I get 12.10 volts when I measure the voltage at the last point ie where the outside wires connect in to the battery bank. At this point the voltage as dropped down to 11.6 volts.

Between this so called last point and the batteries there is a fuse box and charge controller and another fuse before the batteries. There are also on off switches to isolate each circuit. The fuse to the outside lights is 5 amp and the internal fuse to the controller 15 amp. The wire size is 6mm. The amp draw is 2.36 amps. The wire side leading to the various outside lights is 2 mm or 3 mm.

This voltage drop of 12.1 to 11.6 seems to me to be excessive for such a short distance.

If you are wondering what the no load volts is it is 12.54 v that is what the volts rise to if the lights are switched off.

I was wondering if the fuses or the switches are not responsible for "choking" the voltage in some way.

These readings are according to my multimeter. Not the readings on the charge controller which are different.

I would be grateful for any input or suggestions that would help me improve my system and its output.
 
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Old 05-08-14, 05:20 AM
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You may have resistance at every connection in the system. Even the slightest bit of dirt or corrosion can cause real resistance at 12 volts. I would first go through the entire system and clean all connections and contact points and possibly coat them with electrical (dielectric) grease to help prevent corrosion.
 
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Old 05-08-14, 05:21 AM
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While you are measuring voltage in the different places, are the same number and combination of lights switched on?

Something (perhaps the controller) is adding resistance between the point where you are measuring 12.1 volts and the point where you are measuring 11.6 volts. A "charge controller" has some electrical and electronic components in the circuit; it's not a straight wire path through.

I would guess that there are one or more check valves in the controller as part of the control mechanism for solar panel battery charging and the battery lighting circuit. Even though we are dealing with direct current, diodes (rectifiers) are used for this purpose and the typical diode has an immediate voltage drop "penalty" typically ranging from a small fraction of a volt to a little over a volt. Fortunately a diode does not impose a voltage drop proportional to the current draw (it is not a pure resistance); if this is responsible for the 12.1 to 11.6 volt drop, you could draw twice the current (if you did not overload the diode) and this voltage drop would not double.

I do not think that the switches or fuses are contributing to voltage drop.

Incidentally the batteries themselves have some resistance so as to yield the battery terminal measured voltage being higher at no load and lower as load increases.
 

Last edited by AllanJ; 05-08-14 at 06:00 AM.
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Old 05-08-14, 05:35 AM
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Any fuse is going to have some resistance. Think about it, if it had no resistance it would pass unlimited current. The suggestion to clean all contact points is valid. I would check the temperature of each connection point in the system. If a connection has some resistance it will will be warm/hot under load. This may help finding where the resistance is that is dropping the voltage.
 
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Old 05-08-14, 06:23 AM
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What is the distance? That voltage drop is consistent with a 50-foot circuit of 10-gauge (6mm) with a load of 2.36 amps.

Side note: If those deep cycle batteries are lead acid, they shouldn't be in your living space.
 
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Old 05-08-14, 08:10 PM
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I do not think that the switches or fuses are contributing to voltage drop.
At 12 volts everything contributes to voltage drop.

I would say that 6mm (#10 awg) is awfully small to use in that application.
 
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Old 05-08-14, 08:51 PM
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I don't have the data or the tools to do the calculation but when a fuse is handling current close to but less than its rating, the fuse element will be at a temperature close to its melting point which is also means that the fuse element's resistance is somewhat higher than what it would be with everything at or near room temperature. Meanwhile when the fuse is handling a small fraction of its rated current, its resistance is very small (like a few thousandths of an ohm and imposing less than a tenth of a volt's drop) even if the fuse element is made of a not particularly conductive metal especially considering that the length of the fuse element is only a few inches or perhaps even less than one inch.

By the way, to be absolutely correct, at any voltage, everything in the circuit contributes to voltage drop.
 
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Old 05-09-14, 06:37 AM
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Last night we were doing a hospital paging cutover to a new system. I measured a half-volt drop on a 12VDC supply for contact closures across some 20 year-old 66 block punches. Re-punched and all was well, proving that it can happen anywhere.
 
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Old 05-09-14, 10:39 AM
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Thank you for the suggestion of dielectric grease I have never heard of the product but I will definitely esquire about it. Your suggestion about corrosion is a good one I live not far from the Sea and we also have a very humid climate. Here Cars rust, every thing corrodes in some fashion. I will definitely take your advise and go through every connection and clean them one by one.
 
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Old 05-09-14, 10:58 AM
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Thank you for your input. The number of lights are currently at 11 they are on at the time of measure, I was measuring the volts and the amps with my multimeter to see what the power draw of my lighting system is. As a way of measuring consumption of watts vs production of watts.

There is a lot of discrepancies with the measured amount of power produced from the panels to the amount of power used by the system.

This evening after reading your post I changed the 5 amp fuse to a 10 amp fuse to see what would happen. The volts rose by from 11.8 to 11.9

Looking at my system, the switches are are of the toggle type typically found in the auto market sales section. The connections are of the spade type.

I think I must try and get different switches as I am sure the spade's are a choke point for the current flow.

Thank you for your time your post made very interesting reading and left me with some thinking.
 
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Old 05-09-14, 11:08 AM
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The distance from the batteries measured with a piece of string. From the batteries through the switches fuses etc to the charge controller 55cm and from the charge controller through the switches fuses to the termination block is 60 cm. Total 115 cm.

Regarding your side note, I live in South Africa :-) every thing where I live would be stolen in a flash, if it is not secured in some very strong fashion, especially lead acid batteries etc. The batteries are in an enclosed stoep, that would be a veranda or porch out of sight. They people doing the stealing regularly steal the electricity supply cables in the street outside. They are not afraid even of electricity.
 
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