Battery, Solar & Inverter

Submitted: Monday, Aug 29, 2011 at 19:08
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Hi all

Being complete novices at all this solar stuff and hoping to minimise our use of the generator I have questions about all the above that I would appreciate receiving answers from the experts.
Q1. Second battery in the Prado needs replacing. Which is the best way to go? Wet cell, AGM or Gell? Will be replacing 120a/h subject to size. Will only be used to run a 50l Waeco and probably t/v (and inbuilt dvd player) plus set top box.
Q2.Looking at buying a 120w folding solar panel. Will this be sufficient to keep the battery charged?
Q3.Will possible get a 300/600w inverter. Will this be sufficient to run the tv/dvd (draws 70w according to handbook) and set top box (5w)?

Many thanks in advance for your input.

John & Helen
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Reply By: Puma1 - Monday, Aug 29, 2011 at 20:01

Monday, Aug 29, 2011 at 20:01
G'day John & Helen
Why on earth would you bother sucking your batteries dry with an inverter, when you can get a 12v HD tv (lcd or LED) with built in dvd player for about $300 from Kogan? Look at the Magic TV HD set top box with a 1 terrabyte hard drive you can record 2 channels whilst you watch another & that runs off 12 volts too (got mine for$350!

These are some of the things we have bought in readyness for our multi year trip starting in about 5 weeks.

We even got rid of our 240v food processor and bought a cordless one that I'm going to be able to recharge via 12v

All my battery (AA's AAA' 9v etc) chargers use a 12v source too.

Oh and regarding what type of battery to buy - you can't mix types if you are going to parallel them, so what have you already got? 'cos that's what you need, that goes for the a/h too! Keep them "type & a/h" the same, mixing an old battery with a new one is a bit of a no no too!

Recon the solar panels are at the lower end of what you will need I went with 180w and fitted them to the roof so they are working whilst I'm still bludging in bed! So if you are getting portable ones how many hours a day are they going to be open - work it out.

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Follow Up By: Member Al (Sunshine Coast) - Monday, Aug 29, 2011 at 23:08

Monday, Aug 29, 2011 at 23:08
Nigel, To say that you cannot mix "battery types" is a generalisation.
What you should not do is mix batteries of different 'chemistries' as they require differing charging voltages.

Flooded wet cells, both cranking and deep cycle, are the same chemistry (lead-acid) as AGM and can be parallel connected. The AGM has a different cell construction but is the same chemistry. However Gel batteries are of different chemistry and should not be mixed with lead-acid batteries.

Provided the batteries are of the same chemistry it matters not if they are the same ampere hour capacity to be parallel connected. They will charge and discharge in proportion to their relative capacity and be quite happy. Mixing old and new is also of no consequence until one becomes so poor as to suffer from self-discharge whereupon it will also discharge the 'good' battery. This discharge should become evident and a simple test with the batteries separated will reveal the one needing replacement.


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Follow Up By: Puma1 - Tuesday, Aug 30, 2011 at 00:49

Tuesday, Aug 30, 2011 at 00:49
G'day Allan, your right I did generalise type and I did mean chemistry as I saw gel and acid types on the list, I knew someone would step up to the plate and straighten it out.

Re: different battery a/h's from all research I have done and from discussions with battery shop owners/workers and spending many hours (half a day) googling the subject, it is my firm belief that if you are using say a 120a/h and an 80a/h that the 120 will not be able to charge above 80a/h which means it is 33% drained at best, and this in turn will cause the 120 to fail because it is never going to get fully charged. So when the 80 is full charged the 120 is 66% charged, so when the 80 is 50% (half) discharged, the 120 at this point is only 33% charged and it's not doing any good to either battery.
Also running those 2 in parallel you will have a combined max of 160a/h not 200a/h that your theory (of them both giving their all) would give, spend an afternoon gathering all the facts then tell me I gathered mine wrong. In the meantime I'll stick to the same a/h in any that I parallel.

There's some really interesting facts (found another 5hrs googling) on mixing different wattage solar panels, the facts I found seem to go against what many believe to be true. It was a real eye opener, and stopped me from adding a different wattage panel (that I owned already) to the 3x60's mono crystaline's I bought to get used to get 180watts but that's another story!
Cheers Nigel
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Follow Up By: Member Al (Sunshine Coast) - Tuesday, Aug 30, 2011 at 10:55

Tuesday, Aug 30, 2011 at 10:55
Hi Nigel, When did you last find a person with adequate formal electrical qualifications running a battery shop? They have all sorts of commercial and marketing knowledge about batteries but little understanding of electrical science.

The charge (current) delivered to a battery, any battery, is foremost a function of the voltage difference between the source (alternator or charger) and the battery. The impedances of both the source and the battery also play a secondary part in the current. So when two or more batteries are connected in parallel to a charging source and the batteries begin to charge they each draw current in accordance with their internal voltage and impedance. Should the smaller battery tend to become more (or fully) charged, its internal voltage rises to almost the same as the source voltage and current flow to it diminishes or ceases (it is now 'floating') whilst the larger battery continues to draw current until its internal voltage also reaches the same as the source whereupon its charging current tapers off. Both batteries are now at a fully charged state. In practice, both batteries charge at their proportional rate (due to their rising internal voltages) and arrive at the fully charged state together.

A similar scenario takes place on discharge of batteries in parallel. Current initially flows from them in proportion to their relative impedances and as their voltage falls the battery which can better sustain its voltage (the larger one) will deliver more current. This continues with the voltages balancing the discharges and the batteries each maintain the same State of Charge.

Your dissertation of "the 120 will not be able to charge above 80a/h" makes no attempt to explain why this would be so and is incorrect. My expressions are not just 'theory'. They are established and demonstrated electrical engineering principles. All electrical engineers understand and work with these principles every day.

"Water Analogy" does not always accurately apply but it does in this instance and for those who appreciate it here is the example:
Two water tanks, say 1000 and 500 litres sit side by side and are connected together with a pipe which also connects to a source pump. As the pump feeds, both tanks receive a flow (amps) and if one tank tends to rise above the other then its increasing internal head (voltage) opposes the flow and the two tanks rise in level (state of charge) until pump delivery ceases. Each tank has taken a different quantity (Ah) but is at the same level.(proportional SoC). Exactly the same occurs on discharge from the tanks. Each delivers in proportion to its head so that the level (SoC) of each tank is identically maintained.

I have no desire to "spend an afternoon gathering all the facts". I have already spent a lifetime gathering electrical knowledge, some in formal education, some in practical experience, and have established my proficiency. Be cautious of Google Nigel. It provides as much inaccuracy as information..... just as does shops and the bloke at the pub!


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Follow Up By: Member - john y - Tuesday, Aug 30, 2011 at 11:10

Tuesday, Aug 30, 2011 at 11:10
With regard to the Magic set top box Puma mentions ,I have a couple of questions

1/. Is the set top box capable of receiving HD satellite signals as analogue signals are already being phased out in country Victoria .

2/.. Does the box draw much power.The caravan has a 100ah battery fitted which would be running the 12v tv/dvd and an interior 12v light at the same time or would I need a separate 12v power source.

3/. Where do you source the product from?

I have a honda 2.0 generator but would like to use a 12v system at night as my better half is an avid TV viewer and I am conscious of the noise intrusion onothers.

Any information ,suggestions or comment would be appreciated

john y
I will go anywhere as long as it's forward

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Follow Up By: oldtrack123 - Tuesday, Aug 30, 2011 at 15:05

Tuesday, Aug 30, 2011 at 15:05
Hi Nigel
I would suiggest you read Allan's post carefully
IT is 100% correct !!
Your's is another case were a little knowledge can be misleading, if the total concept &/or theory is not understood
The trouble is the extent to which such misundertandings are spread via the net.

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Follow Up By: Member - Bruce C (NSW) - Tuesday, Aug 30, 2011 at 16:10

Tuesday, Aug 30, 2011 at 16:10
Hi Allan, (from the Sunshine coast)
Well explained information there Al and clearly stated too.
Thanks for the info. Nice analogy as well in the 2 tanks.
Cheers, Bruce.
At home and at ease on a track that I know not and
restless and lost on a track that I know. HL.

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Follow Up By: Member Al (Sunshine Coast) - Tuesday, Aug 30, 2011 at 16:43

Tuesday, Aug 30, 2011 at 16:43
Thanks Peter for your informed support.
Thanks also to Bruce and Ken.

It is not my intention to humiliate other contributors but patently wrong information needs to be rebutted. There is much incorrect information published on the internet, particularly on electrical matters and people are led up the garden path. The worry is that those promulgating myths are convinced that they are correct. The real problem is that they are unaware that their beliefs are untrue.


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Follow Up By: Puma1 - Tuesday, Aug 30, 2011 at 16:51

Tuesday, Aug 30, 2011 at 16:51
Thanks Allan taken on board.

John specs to Magic TV are:
Physical and electrical specification:
Dimensions: 325mm (W) x 230mm (D) x 62mm (H)
Weight: 3Kg
Power supply: +12V DC. 100-240 VAC auto-ranging power adapter included.
Power consumption: 20W (max). Intelligent power saving mode is included.

got mine from here:

It's the latest version but you have to source the 12 volt cigi plug for the van as it only comes with a 240volt 'er like a laptop computer. Not hard to find I had one lying around that works fine.

Changed all my lights to LED's too, cost me $1.25 for 2 delivered went from 2 x 10watt to 2 x less than 1 watt same amount of light just less than 1/10th the power consumption
see some here $220 delivered for pack of 2

Search and find them in different colours too

Cheers Nigel
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Reply By: Meggs - Monday, Aug 29, 2011 at 22:03

Monday, Aug 29, 2011 at 22:03

Q 2 Maybe if you get full sun every day. I can just get by with an 80W panel in winter with full sun every day and the fridge load is low.

Q 3 What's a set top box? is that the thing they give to pensioners. Get an LCD TV runs on 12v and cheap as chips now.
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Reply By: Member Al (Sunshine Coast) - Monday, Aug 29, 2011 at 22:51

Monday, Aug 29, 2011 at 22:51
Q1. If the second battery is in the engine bay then definitely not a gel and even the AGM is not happy about heat. So best to be a Flooded Deep Cycle ( Probably what you are calling a 'Wet Cell'). If the second battery is not in the engine bay then AGM would be best as it is more compatible to the charging regime of the alternator and cranking battery.

Q2. The ability of a solar panel to meet your consumption demands is of course dependant on the amount of sun it receives. However it seems most people manage a modest fridge from a 120w panel. This makes no allowance for a TV.

Q3. If the TV/STB draws only 75W then why would you want a 300W inverter?

Now for some consumption estimations, but I'll leave out the detail maths.
An allowance for your fridge would see it consuming about 36 ampere hours in a 24 hour period. (More or less, depending on your usage)
If you watch 3 hours of TV via an inverter you will consume 24ah.
Already you are up to 36 + 24 = 60 ampere hours which is as much as you should discharge your 120ah battery. You now have to replace that 60ah from the solar panel next day.

So like Nigel advised, get a 12v LCD TV and thus avoid the inefficiency of an inverter and the 240v TV. You could just go early to bed or read a book with a LED headlamp. LOL


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Follow Up By: KenInPerth - Tuesday, Aug 30, 2011 at 16:08

Tuesday, Aug 30, 2011 at 16:08
Hi Al

I have not been around for a while

As always your information is very good reading.

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Reply By: westskip - Tuesday, Aug 30, 2011 at 22:33

Tuesday, Aug 30, 2011 at 22:33
Many thanks for all your input. It has been most informative. Hopefully we will now be able to work out our reqirements ready for next year's longish trip.

John & Helen
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