Battery Vehicle Society

Hi All,

I am just wondering about a charging regime for my batteries.
I have the twelve Trojan T1275 batteries from EV_Dub as my battery bank.
My plan is to wire these in three 'rows' of four batteries wired in series to get a 48v 450ah pack.

The Trojan information says to charge them at 14.8v for a daily charge, 13.2v float charge and 15.5v equalising charge. (all at 25 Deg C)

The way I want to use this battery bank is very different to an electric car.
Our house power requirement will be around 5kwh per day.
We will be replenishing this with a 'trickle' from a small generator powered by a stirling cycle engine.
I was invisaging this being around 300W and running continuously 24hrs/day 365 days/year.

My plan was to get a big battery pack so we cycle the batteries only lightly to keep the number of available cycles high and the battery life long.
I was wondering if I might be charging the batteries too lightly too? and and at what voltage I should do this at? as it might be part way between a float charge or daily charge.

We will build a small straw bale building for the batteries and keep them at 20 Deg C to keep things stable.

Any thoughts?

Rob

Hi Rob, you have a fairly interesting setup.

The charging figures for the trojans are exactly that. Charge the battery up to 14.8 volts and it is full, you then float the battery at 13.2 volts if it is not supplying any power. The equalising charge to 15.5 volts should only be done after the batteries have delivered a certain amount of ah's. This is pretty much the same as a gwiz battery pack, but with twice the capacity.

What sort of charge controller are you going to use to charge the pack? I would have thought that with a fairly small continuous charge that a solar or wind charge controller could do a good job. What will you do once that batteries are fully charged with your generator - switch it off, or have the power switched to a dump load or something?

What size invertor are you using to convert your 48 volts to house mains? Have you considered what your peak load might be in your house? I have measured it in ours and I have to say with some combinations we have hit 6kw - but that does include having my electric car plugged in as well

You have to think of a battery as a muscle - keep it well used and excercised and it will work well. Your pack came from an electric car with a powerful contoller which would have hammered the batteries with massive current draws. Do you know how many miles they were driven for? A gwiz with a 48 volt 200ah pack will do on average 7000 miles before the range drops considerably, but some people get alot more than this as they drive them extreemly gently.

Be interesting to see how you get on with them.

Regards

Matt

Hi,
Thanks for the reply.
I am still thinking about how to get this system to work.

I am wanting to get these Trojan T1275 batteries charged from a small stirling cycle engine.

The battery pack might well be a 48v 300ah pack.
If I were to charge these at 59.2v and 10amps that is 592W. If that is left charging for 1 hour that would be 0.592 kwh input. How much of that would remain in the battery and be available again as battery power?

These currents might be out of the realms of electric vehicles but the little stirling cycle engine I will build will be running 24hrs a day 365 days a year and burning fuel from 2 acres of coppiced willow. We will need to add to this fuel with any old rubbish timber too but it is still quite finely balanced.

Rob

Hi Rob,

You would ever be able to put 10 amps in the batteries when they were at 59.2 volts as they would be fully charged. The nominal voltage of a lead acid abttery is 12 volts. As you charge a battery the voltage on it rises as it fills up. You have said the voltage given by trojan daily charging is 14.4 volts. This is the finishing voltage of the charge, so when they reached this voltage you would have to stop charging them. There are two main stages to charging a lead acid battery, bulk charge and finishing charge. The bulk charge is basically stuff as many amps as you can in to the battery until it reaches a certain voltage. As the battery fills the voltage will rise and if you put in lots of amps the voltage rises faster as the battery fills. In the finishing stage you tend to hold the voltage at a fixed value and then let the charging current drop down until is goes below a certain amount, then turn off and it is charged. If you were doing a maintenance charge you would take the voltage limit off at the end of charge and let the voltage slowly rise up to 15.5 volts and then terminate the charge.

Now you do have to take into account the efficiency of the system.
If you were to work on about 85% in charging the batteries, and about 85% when taking the power back out.

So for every 1kwh put in to the batteries you would get ( 1000wh * 0.85 ) * 0.85 = 722.5wh back out.

I hope this helps

Hi,
Thanks for the response.
I have been working on this since I put up the posting.
I have now come up with pretty much exactly what you have said.

We have swapped our freezer this week and that has had a surprising effect on our power consumption.
I thought it was using a lot of power. The last time I checked it it was using about 1.5kwh / day. We have recently been using about 7 or 8 kwh per day in total. Since we swapped the freezer over that has suddenly dropped to about 3 or 4 kwh per day total for the house.

I have had to have a rethink about the power output from our stirling cycle engine. My plan was to have it ticking over 24hrs a day 365 days a year. The final current before switching off would be about 6 amps. (I have chosen to now go for a 300ah 48v pack). For my simple circuitry to sense the drop to 6 amps it has to be higher in the first place. Working on around 10 amps and up around the 60v mark that is 600W so need to go bigger on the stirling engine and run it for a shorter time. I am now looking at something like 1kW mechanical power from the stirling engine.

Edit: I ordered a load of zener diodes, comparators and bits and pieces today to make up the circuitry that triggers the different charging points.

Rob