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Month: September 2018

Solar charging on a rainy day

Solar charging on a rainy day

When I logged the second solar charge last week I had the computer plugged into mains power and the homebrewbattery inverter was switched off. There was basically no load on the battery, and that was visible in how flat the battery voltage curve was when the charging was finished. Today I raised the stakes and left the battery powering the laptop that was performing the logging. On top of this it was a rainy day with substantial cloud cover. Would the charging keep up at all, or would the laptop drain the homebrewbattery too far?

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Pop!

Pop!

Oops!

I’m having a bit of difficulty getting my packs balanced to start an experiment to see how well they remain that way. I think an easy way to do this would be to connect the 7 packs in parallel and leave them overnight to exactly equalise voltages, but this will require some new “bus bars”. I grabbed a wire this evening and held it roughly in place to check what length I need, and then POP, FLASH!

I’d absentmindedly touched the bare wire onto my pack connectors for two packs, shorting them out and instantly blowing all their fuses.

My battery project time today has been used up with replacing 20 fuses.

Characterising the solar charge

Characterising the solar charge

I was excited earlier this week when my battery fully charged in a day, but it left me wondering how that charge had progressed. The solar panel is only leaning against the house for now, and it faces west. I wanted to watch the charging activity, but frustratingly have to spend the sunny part of the day away from home at work.

A full battery doesn’t need charging, and so I powered my laptop from the homebrewbattery for about 4 hours in the evening while researching how to log charging data from my PCM 60x charge controller. I estimated about 120-150 Wh of energy were used. In the morning I left a computer logging the PCM 60x every minute.

The battery was fully charged by 2pm! This is surprising since the solar panel points to the afternoon sun – and this is actually “visible” in the data because the 265 W panel only reached a maximum of 90 W before it scaled back due to the battery being at maximum voltage. By numerically integrating the shaded area under the charging power plot, I discovered that the battery took 135 Wh of energy to be charged. This is spot-on my estimate of how much I had flattened it!

The obvious conclusion from this successful test is that I need more cells to make my battery bigger. Back to the cell-testing regime for now…

It charged!

It charged!

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I wasn’t sure whether the small 0.1 A charging current yesterday afternoon was due entirely to the shade on the PV panel or whether it had something to do with minimum input requirements for my PCM 60x charge controller. On paper, the 265 W solar panel should be able to charge my 500 Wh battery in two hours or less.

I left it all connected while I was at work today, and checked it this evening. The battery voltage has risen from 26.1 V last night up to 28.3 V! That’s quite close to the 28.6 V cutoff that I programmed into the charge controller, which suggests that the solar panel essentially gave me a full charge. How exciting!

Solar panel connected

Solar panel connected

Completing the first stage of my battery was a great feeling, but it has been sitting underutilised for a few days. My iSDT Q6 smart charger takes a 7-32 V DC input (nicely covering my 24 V system) so I’ve been able to power my cell testing from the battery. This bootstrapping is nice, but it has been slowly draining my battery and I’ve been reluctant to charge from the grid – the main point is to store sunlight! This weekend I’ve “plugged-in” to the sun.

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A few months ago I bought six QCELLS Q.PRO-G4.1 265 solar panels on ebay for a fantastic bargain. At 265 W each these panels give me just over 1.5 kW of electricity production – way too much for my 0.5 kWh battery. For now, I’ve wired up a single panel to give a nominal 0.5C charge (2-hours full charge if the sun is bright).

Along with the solar panels I bought a second-hand MPPSolar PCM 60x charge controller. Again, this is oversized for my battery at this stage – it can put out a 60 A charging current – but I’ve dialled it down to 10 A. This is 1 A per cell in my current battery, which matches my testing regime and represents my “upper limit” in the design parameters.

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For now, the solar panel is leaning against the back of the house. With only 9.23 A of short-circuit current from the single PV panel, I have taken a temporary shortcut and used 2.5mm twin-core-plus-earth (2.5 square mm cross section area for each of active, neutral, earth – normal internal wiring for household power sockets) to connect the panel to the PCM 60x charge controller. This wire is rated for 20A, and is currently routed under my house so does not have any UV or thermal load to deal with.

It took a lot of fiddly time to measure the cable distance, attach MC-4 connectors, triple-check polarity, connect into the PCM 60x terminals – and by the time I’d finished the sun was low enough to leave my PV panel perfectly in the shade!

Despite this, I was charging at 0.1 A! Everything seems to be working perfectly, and now I need to wait for some sun.

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Cells, Packs, … finally a Battery

Cells, Packs, … finally a Battery

I finally have the first modular instalment of my battery complete: 70 cells in a 7s10p configuration made of seven “packs”. The pack construction has been a bit slow, but along the way I’ve built some construction jigs and optimised the process. I think I could do the next set of packs in less than half the time!

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First stage of the battery wired up and ready to go. It is almost exactly 0.5 kWh in capacity.

These are only half packs, because my design is to have 20-cell packs as the basic battery building block. This means that they don’t “stand up” as intended in the final design – but I don’t have the battery rack built yet anyway, so I’m happy for them to lay flat on their sides for now.

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The battery looks great from every angle.

In place of a proper battery management system (BMS) I’m using a hobby charger that can do a balance charge (monitor the voltage of each of the 7 packs individually). This is why each “join” jumper between the packs needs to have a separate return wire.

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Cheap inverter for now, but it powered the lights fine for this photo!

In keeping with the “start cheap and grow” philosophy of this project, I picked up a second-hand 24 V inverter for $50. It has a rated output of 600W, but is a modified sine wave inverter which is not great for  sensitive loads. As a proof of principle I used the 240 V power from the inverter to power the lights for these photos. With my current battery size I can’t even use the full 600 W as it would draw more than 2 A per cell (I’ve designed and tested for 1 A max).

It was a great feeling to finish the wiring harness and hit that switch. There’s a long way to go before the battery is really useful, but it is now ready to power things and let me move into a fun new stage of testing and design. I can return to cell extraction, and while I’m accumulating the next 70 good cells I can tackle the next big challenge: charging this battery from the sun!