Battery and solar help!

Just to follow up, we never let the fridges go empty, every time a beer comes out it is replaced with another, it helps the fridge maintain temps easier.

Love your way of thinking Cruiser

Very in depth! Thanks. Those links are great.

Hello
I have read your link Electricity for camping which was great.

I have a couple of queries if you could help me please.

1.My Engel states 2.5 A(max), so if it is cycling at 50%
that would be 1.25A per hour therefore 30A over a 24hr period?

2. TV- 4.16A over say 2hr =8.4A per 24 hr?

3. Sat box- 2000mA over 2hr =?

4. Cpap running through 90W ac adapter which @24V ------3.75A say 8 hr =30A??? I have this plugged into a projecta 300 PS watt inverter.

I have 100 W solar panels with a PWM module feeding a 120 Ah AGM battery in a ARK box.I have another 100Ah batt which services the led lights etc in the van.
How much can I expect from these panel on a good day and what panel would I need, to replace and top up the batteries with this approx 80 Ah per day drain?

I thank you for your help and apologize to others for possibly re-hashing old material.

Hi Geoff,

This does fit well under Damien's header, but if it expands as it well may, it may be better to start a new thread.

"1.My Engel states 2.5 A(max), so if it is cycling at 50%
that would be 1.25A per hour therefore 30A over a 24hr period?"

Your maths is indeed correct.

May I correct one thing though - many folk fall for this and it really irks after a while - An amp (A) is a measure of the rate of flow of electrical charge. ( It's actually a flow of one coulomb (6.241 x 10^18 electrons) per second, but let’s not get too hung up on the physics!) So --- we don't refer to amps per hour, any more than we'd refer to kph per hour. (I'm not being critical of you specifically, or aiming to prove I'm a boring pedant! but there are usually over 1000 people logged onto this site, and using incorrect expressions leads to lots of confusion. )

If your fridge is running about half the time, and draws 2.5A when running, then the average consumption is indeed 1.25A. The total demand we express in amphours (Ah), i.e. flow rate x time and so in 24 hours your fridge will call for 1.25 x 24 = 30 Ah.

Your TV - I suspect that it is running from a power adapter labelled 4.16A. If that's the case, then that's probably the maximum that the adapter can supply, rather than what the TV draws from it. I'd expect the TV to in fact draw less than the 4.16A, let's say 3A for 2 hours = 6 Ah.

The sat box. 2000mA = 2A. Again, I suspect the box itself draws less than that, probably about 1A rather than 2A, so in 2 hours that would be 2Ah.

The CPAP - I've recently been around the CPAP loop myself and am still looking for a human compatible system! The adapter is capable of delivering 90W at 24 volts. If it's the machine I think it is (Resmed), there is an alternative 60W adapter which will handle the pump plus humidifier, the 90W adapter being intended to also run a heated tube. If I'm right then you have some options - the pump itself will require a bit less than 30W, the humidifier (which can be turned off) will account for an extra 30W and the heated tube (also can be turned off?) another 30W. Each of those 30W loads corresponds with a current of 2.5A at 12 volts, plus 20% to account for inefficiencies in the inverter and power adapter. That comes to 3A, 6A or 9A. In 8 hours, 24, 48 or 72 Ah. The options to turn off the humidifier and heated tube are important!

Your 24 hour energy budget in rounded figures is
Fridge 30 Ah
TV and satbox 10 Ah
CPAP 25-75 Ah
plus a little (say 5 Ah) for lights, charging torches, phones, etc etc

If your battery is to have a decent life span it shouldn't be run down more than 2/3 of capacity and should be recharged asap. On that 2/3 basis, your 120Ah battery can deliver about 80 Ah. That's enough to handle the fridge and CPAP with humidifier but not heated tube.... and no TV!

How to recharge the battery? Under optimum conditions your 100W panel will provide almost 6 amps with your present PWM controller, or almost 7 amps with a MPPT controller. If we assume 7 hours of good sunshine per day, and keep the panel reasonably aimed at the sun, that's about 40 Ah per day with the PWM, about 10-15% more with an MPPT controller. You need at least 200W of solar capacity to meet your minimum requirements. To have any flexibility you also need to at least double your 120 Ah battery capacity. (It may be possible to make use of the other (100Ah) battery.) You could also provide some charge to the batteries from you vehicle - the balance between solar when stationary and alternator when driving needs further consideration.

As you obviously appreciate Geoff, there's a lot to go into here. Usually the fridge is the main consumer, but adding CPAP adds a whole new dimension. You do need more battery storage and more charging ability. Maybe you might consider the Lithium batteries referred to by Alistair, but $$$$......

Hope that helps

John

My post crossed John's and so some effort was wasted.

Excellent explanation x 100, Alastair D, no info is ever wasted.

Good one Alastair.
Cannot imagine why anyone would take issue with it.
But tell us about your LiFePO4 batteries..... What capacity are they? What did they cost? What is the charging arrangement? Can you provide a link to their supply source please?

Alan,
I bought my batteries about a year ago from EVworks in Perth. Their primary business is electric vehicles but seem to have developed into a good source of LiFePO4 batteries and associated gear.

EVworks

I purchased from them as when I first started researching this option the principal of the company Tim Brunner answered my many questions with facts and figures and not hype. Their prices were quite good and seemed to have come down a bit since then. I suspect that they will continue to do so as the demand and usage of Li batteries increases.

The early Li batteries had significant problems but now the new variants seem stable, reliable and more a practical option.

I bought 8 160AH 3.2v cells and configured them as a 320AH 12v battery, ie 4 parallel pairs of cells in series. This actually typically gives a battery of 13.2v under load. If you look at the following link you can see that the discharge curve for these batteries is remarkably flat and they drop off very quickly when near the limit.

Graphs

I have spent quite a lot of time reading and playing with the batteries and have now got a good feel for them. I have built a microprocessor based data logger and cell monitor which will become a permanent part of the install. Not necessary but just part of the fun of playing with technology.

In practical terms the batteries hold their charge very well, love to provide high current, which is why the electric vehicle people use them, and charge quickly if you do it right. The big difference to lead acid is that the charge/discharge curve being flat means they provide an ideal energy source and they accept almost full current right up to about 95% SOC at which point the voltage rises fast. You must terminate the charge ideally around 3.8v per cell and certainly before 4.2v otherwise they will be damaged. This is quite easily done and most existing chargers can be used with a little care.

Smart chargers with auto reconditioning, desulphating cycles etc must not be used unless you can turn it off.

I will not go on but am happy to provide more info if you send me a member message.

In summary my cells and bit to go with them cost about $1800. This gives me 320AH nominal but a real 260AH if I go to ~20% SOC which can be done with no penalty on life. Their total weight is 60Kg.

My AGM alternative was to use 2 x Lifeline 255AH batteries to give about the same usable capacity going to 50% SOC. The weight was about 140Kg and cost about $2300. I know I could go for cheaper AGMs but I want reliability and a decent lifetime. I consider I am way ahead as I can expect a 10 year life from the Li batteries if I treat them well.

One aspect of the Li batteries that is good but presents a hazard is that their ability to provide high current means you must ensure good insulation against shorts. Whilst a lead acid will provide a big flash these batteries are in a different league. They just love to discharge !!

Happy to answer any questions via member msg.

cheers

Many thanks for that Alastair. You gave me just what I was wanting.
My current pair of 110Ah AGM's are still holding up but I will start researching Li's ready for when they inevitably fail.
As an electrical engineer I understand about the need for fault-current limitation and take your advice onboard.
And thanks for your member msg invitation. I may have questions as I research and plan.

With regards

And my post crossed John's and Alistair's, so again, some effort wasted.

However in our various ways we have come up with similar numbers and suggestions thus proving it is not an exact science but broad estimates are useful. Unplanned teamwork !!!!!!!!!

Thank you John and Alastair :-)

I'm not sure Alastair will be happy having his mind cast in the same light as mine, but hey, if we have been of any help, then we have served a purpose.

I'm a little concerned that some posters have come up with a seemingly definite figure of 6 amps or so for the fridge when it's running, whereas the apparently authorative website I looked at said 3.3.

6 amps makes a considerable difference to my estimation, and reinforces Bantam's opinion that 200amp-hours of batteries are needed.

Damien really needs to measure the current drawn by his fridge and either get back to us or substitute his figures in our calculations. I'd be interested to know what the real figure is.

Cheers

Frank,
The 6A comes from measurement not data sheets. It was this figure when running when I bought the fridge in 1972 and is the same now. Many of the manufacturers quote average currents, so if the fridge typically runs 50% of the time they will cite 3A average.

My new Bushman fridge is similar and I measured the current when I bought it.

cheers

That second line should read 200watts of solar.

You should install a "redarc isolator" to stop fridge battery drawing power from your start battery.
The solar should be hooked up directly to the fridge battery. I still disconnect solar at night because the MPPT controller still draws power from the battery, its not much but when we are in one spot for a few days every bit counts.

Rays outdoor 120w Powertech solar panels come with a built in charge controller, plenty of leads, Anderson plugs and padded bag@ $600.
I was lucky enough to purchase a brand new one from a retired bush tour operator for $300. I has never let me down in 3 years.

I have installed a Ctek 250s Dual for that purpose right beside 2nd Bettery which is far away from start battery and it can charge from both sources separately or indivually,has a MPPT controller built in and isolates the main battery. bit Xy but redarc have a similar unit but even more expensive.

Not sure if this is relative info for you,
The way you use the fridge is important in preserving battery power.
A empty fridge or near empty fridge will use more power than a full fridge.
Try not to load your fridge with many food items that are at room temps at one time, for example beer.

Here's what looks to be a good battery set up using the Ctek as 178 has mentioned
http://youtu.be/Hy0j6hifJJM

The ultimate set up
http://youtu.be/igSw_wqNWMc

Damien,

re "Can I still charge from the car alternator AND the solar panel at the same time? Would I hook up the feed from the alternator to the the solar input on the Mppt or straight to the battery?"

Yes you can charge from both simultaneously, but.....

As Cruiser mentioned you need switching in the engine bay to disconnect the feed to the trailer when the engine isn't running, otherwise your cranking battery will be delivering power to run the household. Cruiser's Redarc isolator is one such and a good one. The supply from the alternator should be fed directly to the battery, not connected to the input of the solar controller. (By directly I really mean via a fuse. A fuse should ALWAYS be fitted right at any battery to provide protection.)

The Ctek charger mentioned is a combination dc-dc charger and mppt solar controller in the one box. My understanding is that it does not provide both functions simultaneously, so, while everything can be left connected, you won't get the benefit of both sources.

My preference, to get the benefit of both sources simultaneously, would be to use a dedicated MPPT controller for the solar, and initially a direct feed to the battery from a controller in the engine bay. If this proves inadequate, I'd increase the size of the cabling from up front and probably add a dc-dc charger close to the trailer battery. The extra battery recommended by Bantam would also be a very high priority. The discussion above regarding Lithium batteries is very relevant too ---- All of this stuff provides excellent opportunities for spending money!

Cheers

John