Help Designing Backup Power System for Home Use in Africa

Re: Help Designing Backup Power System for Home Use in Africa

mocha wrote: »

I need a reliable system as a solution, but must balance that with cost since for many people it will be a prohibitive cost. So any opportunity to purchase used or salvage, or purchase cheaper components from China or elsewhere would be welcome.

Obviously, I cannot help you with sources in Africa--Sorry.

But for batteries, do you have access to used forklift type batteries (near a port or warehousing district)? User "adas" has been pretty successful getting used/scrap forklift batteries, removing/bypassing failed cells and using the rest to run his off grid mfg. operation in Hawaii. Do a search on his name for threads he has started.

We can start with a 1500Watt and 6000Watt optional configurations for the charger inverter. I don't want to be concerned about incorporating a solar array at this point, but as an upgrade option. For the battery banks I want to consider one configuration of 200Ah at 24V which can be stacked in parallel to produce more capacity.

Those are pretty large inverters for relatively small battery banks. Nominally, we would be suggesting using 200 AH of 24 volt battery bank for every 1kW of inverter output capacity for a "reliable" system.

To get an idea of the capabilities of such a system (200 AH @ 24 volts for a 1kW output inverter). Say they use an average of 500 watts of energy and the battery bank is only discharged by 50% (for longer life--and/or if you use salvage batteries with less than new capacity):

200 AH * 24 volts * 0.50 max discharge * 1/500 watt average load = 4.8 hours of backup power usage per cycle

Does that seem like a "balanced" system design for you?

Things to consider are that the batteries will be used in an environment where the average temperatures can exceed 90 degrees F for three months of the year and gets hot and humid for other parts of the year. My concern is humidity and/or water evaporation in the batteries and the effect on battery life.

Forklift batteries tend to use more water than other types--So, you could very easily have significant water usage costs (one gallon ore more per month for a "small" system?). The standard is to use distilled water, but using filtered rain water (after the roof has been first rinsed of dust/bird droppings/etc.) can be used as a source in regions with poor access to distilled water.

Using well water (without distillation or de-ionizing filter) will probably give the batteries a pretty short life.

I'm asking for help in identifying the correct components and model numbers for the charger/inverter and batteries and where to find the best prices. I would also welcome your feedback and caveats based on your experience putting together similar systems.

Flooded forklift batteries are going to be the most rugged--But even those types are not going to like being discharged to "dead"--And other typically available batteries (truck batteries are used quite a bit in some places) are going to like being deep cycled even less. This gets into how to protect your investment in batteries from people that otherwise might not care if they leave the system on/drawing heavy power until the batteries are dead... Do that more than a few times, and you will have a dead battery bank.

Adding a Battery Monitor (Victron is a good European brand) would be nice to let people see how much energy is in a battery bank--But they will probably still ignore its readings and still take the battery bank dead when needed.

Some Battery monitors have alarm outputs that could be wired to an "inhibit/remote on-off circuit" for the inverter--You could setup your batteries to self protect. Downside is Battery Monitors are not cheap, and people will bypass the alarm circuit if they need power.

You could use a simple electronic circuit to monitor battery voltage and cut the inverter if the bank voltage falls to ~21.0 volts (temperature and load dependent) for more than a few minutes...

But, battery monitors and simple voltage controlled switches all have their accuracy issues.

So--as you can see, my concern is the battery bank investment. If you "lease" the batteries to the end user, they have no reason to take proper care of battery bank and avoid over discharging (assuming you take care of the battery watering and maintenance).

The normal solution would be to sell the batteries to the customer so if they miss-use the battery bank--It is money out of their own pocket.

And if you are limited to "truck" batteries in your region--They are even less rugged regarding deep discharge and you may only get 1-2 years from such a battery bank (or weeks if a customer abuses the bank).

What are your thoughts on the issue?

-Bill

Re: Help Designing Backup Power System for Home Use in Africa

mocha wrote: »

Thank you for your response.

To clarify some of the logistical questions raised, I live in the USA so the idea is to source all the components from here or elsewhere. So issues like access to used truck or forklift batteries should address the US market. In other words, where can I get used batteries here in the USA? I will definitely research posts by "Adas" and the work he has done with salvage parts.

Shipping batteries is going to be difficult (heavy, filled with acid, and for used batteries, if they are in shipment for over 1 month or so, they will start to sulfate (flooded cell/fork lift batteries should not sit for more than a month between full charges--AGMs can go upwards of 6 months, but they are more expensive and possibly not as rugged for these applications).

I was hoping there was a built-in charger controller for the inverter that would handle battery charging without having to invest in a separate circuit. For example, set it to a 50% discharge depth then shut off when the threshhold is exeeded. So when the inverter specs say it has a built-in solar charger controller this does not control the inverter output?

There are quite a few inverter/chargers with automatic transfer switches--And many of these inverter/chargers have some pretty nice programmable battery chargers (is more difficult to get a programmable stand-alone battery charger).

However, I do not know of any that use a Battery Monitor (current shunt+timer) to estimate battery state of charge and let you set a "shutdown" value (i.e., 50% SOC). It should be possible--and probably some of the inverter/chargers have enough hardware/brains to do that--But they have not (again, to my limited knowledge).

There are a fair number of inverters that do have the ability to shutdown on battery bus voltage--But I am not sure how they exactly implemented (i.e., 5 minute/30 minute timers, a few may have current offset estimates). The higher end inverters are going to have more configuration options (but cost a lot of money too).

There are quite a few inverters that will auto control a genset based on battery voltage/time.

Does a solar charger controller provide overcharging limits on AC charging of the batteries? I like the idea of employing a circuit for self protection of the batteries, but I would have thought this would be integrated with the inverter.

More or less, all of the Solar Charge controllers today each behave as if they are the only power source on the battery bank. And respond according to the voltage/time curves they observe. A few can have the controllers coordinate operation (all in bulk, all in absorb, etc.). And a couple have data buses to share information and control--But at this point, there is not much that you can adjust down deep in the programming.

Talking with folks at Midnite (boB, Robin, Ryan/Halfcrazy) or calling up somebody at Midnite may give you more ideas/options. The other companies (Outback and Xantrex/Schneider) certainly have capable hardware and software--But you may have difficulties getting "off book" type of answers.

This is a very helpful equation. So in my case does that mean a 4-way battery bank (200Ah @24V each string) could only power a sole 1.5 KW refrigerator or deep freezer for only 6.4 hours? Somehow that does not seem to be a very economical solution. We would need additional battery power just to be able to turn on some lights. So I must say it doesn't seem to be a very balanced system.

A refrigerator/freezer usually will only run 1/3rd to 1/2 of the time--So they will "run" 2-3 longer than their average operating Watts would indicate:

1,500 WH per day Fridge / 24 hours = 62.5 watts average power

But its operating Wattage (to run the pump) may be ~120 watts. And it can draw 400-600 watts a couple times of day if it has automatic defrost heaters.

Normally, we assume a refrigerator that is not opened could go ~24 hours or so without power--So it is up to the customer to decide if they want to pay for 10 hours of backup power or not.

And--that is the problem with batteries... The do not really store that much energy for their size/weight/cost.

-Bill