Vanistan - a minimalist diy camper - Page 3 - Ford Transit USA Forum
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post #21 of 106 (permalink) Old 12-10-2016, 02:28:PM Thread Starter
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Originally Posted by surly Bill View Post
I always thought 50% was the safe and long-lived amount of drain you wanted.
so you need a 200 ah battery if you want to use 100 ah?

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post #22 of 106 (permalink) Old 12-10-2016, 03:01:PM
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Yep, but then you'll want to have ~ 200 watt solar panel. And a 20 amp controller.
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post #23 of 106 (permalink) Old 12-10-2016, 03:29:PM
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I can't remember Orton's controller, but he could probably max out a 30 amp controller with his panel. MPPT (vs PWM style) will squeeze a couple extra amps out at best (for my-sized panel) but that could be crucial. Most panels are 18 volt or higher. The higher the voltage, the more difference a good MPPT will make. A super-cheap MPPT may make no difference, and possibly be worse than a comparably-priced PWM.
I have a 32 volt 300 watt LG solar panel that states output is 9.4 amps. The solar controller is a 15 amp max. output Morningstar Sunsaver MPPT.

The MPPT will convert the high voltage panel output to your approx. 14 volt charging amperage. So in ideal conditions the panel will produce 300 watts. The MPPT controller does not create power but it does change the high voltage and low amps to lower voltage and higher amps. Watts in = watts out. 9.4 amps x 32 volts = output amps x 14 volts. So output amps at 14 volts would be about 21.5 amps at 100% efficiency. A PWM controller does not harvest the higher amperage available from a higher voltage panel.

In real world with incorrect orientation to the sun and hot conditions where the solar panel is installed it may be about 70% efficient. 21.5 amps x .70 = about 15 amps. Sunsaver controller may be slightly undersized for the panel but is smaller and cheaper than larger controllers. Did have a discussion with a Morningstar technician and he explained to me that even if the controller is slightly undersized that the total output would not be materially changed. His comment was the panel starts at low amperage in the morning and peaks to the maximum amperage around noon and then declines to low amperage at night. The only lost energy with a slightly undersized controller would be at the top of the bell curve around noon which is minor compared to the overall energy. All you lose is the very little energy that is above 15 amps around noon.
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post #24 of 106 (permalink) Old 12-11-2016, 02:52:PM
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somehow missed this earlier. that would be awesome. i am looking to do a ~100 ah DC system. any advice is appreciated
You can use our 8020 tower brackets to install 2 Renogy 100W Eclipse panels on the roof and retain space for a vent fan. Then place two group 27 (Full River makes a 6V group 27 battery) batteries for ~200Ah under the van in our battery box.

For our van we are planning on a 1000 watt pure sine inverter from Renogy and a Ctek based solar/alternator/shore charging system. Best Renogy prices and free shipping can be had on Amazon.

More info and photos in our build: Considering a 130WB medium roof for sports wagon + R&D work
We also built out a Sprinter: http://sprinter-source.com/forum/showthread.php?t=27822
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post #25 of 106 (permalink) Old 12-11-2016, 08:51:PM
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I plan to muddy the MPPT vs. PWM waters a bit here.

The main advantage that MPPTs have is that they can match the output impedance of the solar panel to the input impedance of the batteries at any particular point in the charging profile. This is important because you can extract the maximum energy out of the panels when impedances are properly matched. Since the panel impedance changes with available light (goes down as more light is available) AND battery impedance changes with SOC, the MPPT can change that impedance matching on the fly.
This means that it can extract more available energy in low light situations (off axis, cloudy, etc.) than the PWM. Exactly opposite to what might have been suggested earlier in this thread.

A PWM controller can't do this. All it can do is limit excess available energy, like when it's bright out and your battery is well charged.

Here's one paper that discusses this in a bit more detail. It primarily focuses on temperature variation, but it does briefly discuss irradiance.
https://www.victronenergy.com/upload...WM-or-MPPT.pdf

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post #26 of 106 (permalink) Old 12-11-2016, 10:56:PM Thread Starter
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is there any way to link 3.8V batteries to make a 12V? Is linking 3 3.8V batteries in series to make 11.4V sufficient for things that run on 12V ?

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post #27 of 106 (permalink) Old 12-12-2016, 12:02:PM
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There is a lot of myth, superstition and just plain bad info about batteries floating around the Internet. (I'm not referencing any comments on this thread.). So, for example, in reality, you can charge your batteries however you like and they will work to a point. However, if you want to maximize capacity and life, unless you want to perform a lot of R&D, your best bet is to design a charging system that closely follows the battery manufacturer's charging recommendations. If you want to charge your batteries as quickly as possible (important for maximizing solar systems where the sun is only available for a given amount of time each day), you also need temperature control (most batteries can be charged faster so long as you monitor them for overheating).

Regarding two of your questions:

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so you need a 200 ah battery if you want to use 100 ah?
Yes, if your load is similar to the load used in the AH rating (search Google for "amp hours battery standard" to learn more) AND you want to maximize battery life. If your load is lighter, you will generally get more runtime. If heavier, you will get less. Depending on your device's sensitivity to low voltage, you can draw your batteries down more than 50% of their AH capacity - possibly in exchange for some battery life - consult your battery manufacture or just take your chances.

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is there any way to link 3.8V batteries to make a 12V? Is linking 3 3.8V batteries in series to make 11.4V sufficient for things that run on 12V ?
Without a very inefficient transformer, there is no way I know of to get exactly 12V with 3 3.8V batteries. Probably the 11.4V you get from wiring them in series is sufficient for most devices, but you won't get as much runtime as you would with a 12V system. Many devices have a low voltage cut-off or simply malfunction below a certain voltage. When you apply a load to a battery, the voltage drops. Wiring runs also add some voltage drop. So, starting with a lower voltage, you will hit the low voltage cut-off earlier. If you can get 4 of those batteries, you could get 15.2V. That also would probably work, but you'd have to be really careful to check each device to see what it can handle.

A lot really depends on your real life loads. For example, right now, I'm charging my laptop via my inverter. My 85AH battery voltage is 11V (I drained it last night running my heater fan at 7A for 10 hours) and my inverter seems happy. However, even with a fully charged battery, I still can't run the compressor on my overhead A/C. When the compressor kicks on, the battery voltage drops low enough that the inverter turns itself off. If I crank the van, the van's chassis alternator and battery allows the inverter to run the A/C on high.
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post #28 of 106 (permalink) Old 12-12-2016, 01:34:PM Thread Starter
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thanks ptjezza!

i am trying to design the simplest system possible. DC only. i want to avoid having a shore power option, so i need to find a battery/panel combo that keeps the battery topped off.

in my dreams there is a way to charge the house battery with the vehicle when i am driving. there must be a way to do this simply? plug into the 12V thing on the van only when i am driving?

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post #29 of 106 (permalink) Old 12-12-2016, 03:01:PM
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Originally Posted by molly View Post
thanks ptjezza!

i am trying to design the simplest system possible. DC only. i want to avoid having a shore power option, so i need to find a battery/panel combo that keeps the battery topped off.

in my dreams there is a way to charge the house battery with the vehicle when i am driving. there must be a way to do this simply? plug into the 12V thing on the van only when i am driving?
Molly if you want to ride the wind as you say, here's what I've come up with. Have yet to implement but goals identical to yours: No shore power, no solar panels (don't want to cut my roof), able to rely solely on alternator for recharging. Able to run all electricals for five days or more without having to recharge. When necessary to recharge able to be done by the time we reach next destination (usually within 4 hours driving time or less).

What's referred to as Battery-to-Battery charging of a Lithium battery system. Lithiums allow depletion of up to 80% or more without harm. They recharge quite quickly not requiring three stage build up. One slight drawback is you could someday require the money such a system costs to put your children through college or pay a veterinarian to solve a pet's health problem. But, whatever, once the money is gone you'll hardly even miss it.

For being simple there is a boatload of stuff to solve but having to also throw in for solar and/or shore power would make it downright un-fun.
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post #30 of 106 (permalink) Old 12-12-2016, 03:07:PM
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where can i got to find out more about this system you describe... it would be perfect.

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Molly if you want to ride the wind as you say, here's what I've come up with. Have yet to implement but goals identical to yours: No shore power, no solar panels (don't want to cut my roof), able to rely solely on alternator for recharging. Able to run all electricals for five days or more without having to recharge. When necessary to recharge able to be done by the time we reach next destination (usually within 4 hours driving time or less).

What's referred to as Battery-to-Battery charging of a Lithium battery system. Lithiums allow depletion of up to 80% or more without harm. They recharge quite quickly not requiring three stage build up. One slight drawback is you could someday require the money such a system costs to put your children through college or pay a veterinarian to solve a pet's health problem. But, whatever, once the money is gone you'll hardly even miss it.

For being simple there is a boatload of stuff to solve but having to also throw in for solar and/or shore power would make it downright un-fun.

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