Electrathon America Forum

Nitoragro · Senior Member

Mixing yellow/red tops

ProEV · Senior Member

Ryan,

Full marks for paying attention to what other teams are doing and trying to understand why.

What a fasinating question. We know that batteries can be designed to be better at different tasks. A starter battery is optimized for power. A deep cycle is optimized for energy. So when asked to deliver high amps, a starter battery should be able to supply more total energy than the deep cycle under high amps. And when asked to deliver low amps, the deep cycle should be able to supply more total energy than the starter battery Under low amps.

Basic electrical theory says current will be the same through out the electrical circuit, so the two batteries in series will experience the same current loads. The starter battery is 'stiffer' meaning that it's voltage will sag less under load. So if the starter battery sags to 11 volts under a 50 amp load and the deep cycle sags to 10 amps, then the starter battery is providing more of the energy, 55 Watts to 50 Watts. This means the starter battery is now more discharged and it's no load voltage will be less and under lighter amp loads, the deep cell will be a higher voltage and provide more of the Watts. This would mean each battery would be doing more of the work in the situation that they are optimized for. So I would say "yes", this might be an advantage.

But...your pack is empty when just one battery becomes empty, so it is most important that the cell capacity works out so that they hit empty at the same time. This method does seem to self balance some but capacities need to be close.

Also, keep in mind that testing says there is enough variation in Optima battery capacity that a brand new good battery of either type will be better in all respects that a brand new bad battery of either type. Also that age and temperature hugely effect capacity. And that it takes a number of 'break in' cycles before the battery delivers it's best capacity.

Nitoragro · Senior Member

On my #80 car I use 3 brand new identical batteries 35ah 12v batteries(Each only drained twice). At the end of the first race on Saturday (at Middleton) they all read differently ranging from 12.02-12.15. I think that is within a normal range. But after the 2nd race we read 11.85 in the lowest one and 12.2 in the highest. Even with this large range we were still running at full speed at the end of the race and to me it felt the same driving. So last year when the batteries were less then 12v our car won't go full speed so how come with that one "dead" battery we were still able to run at full speed? These new batteries we have never run dead in a race so I don't know what is their "dead". But if we used the two optima red/yellow and one we're to "die" would that result in a dead car? If we ran 2 batteries and one was a dead battery that read 11.7 and the other is partially charged at 12.3 would it run normally since the motor is still getting 24v? Would the full battery just be drained a lot faster since more since it has the charge the dead battery a little and run the motor?

mhodgertt · EA President

I always tell my kids to read the batteries with a slight load on them. So give the car SLIGHT throttle with the car held tight so it will not move, and read the volts on each battery then. This tells you the actual voltage the battery is putting out when under load. If a battery sits for a couple of minutes it gains a surface charge that is not really telling you much of anything. Do you have any type of battery management system on your car? I know my 2 yellow Optima batteries start out the race at about 26 volts and they go to about 25 by the second/third lap but hopefully at the end are at about 22.5-22 volts so that I am just starting to slow down. The last lap I like to only be able to go about 90% of the top speed on the straight a-ways as then I know I got all of the goodies out of the battery but did not try to get too much and slow down enough that I get passed easily. I also do not slow down any more on corners than I absolutely have to as the acceleration is what kills my batteries. I do not have regenerative braking as I try to never touch a brake and will coast a little on the way to going into a corner if I need to slow down. But on most of out tracks--even the very tightest ones--if I am going about 26 on the straights I try to hold 22 or so on corners so there is not speed to regain. But then there are very few cars that will corner as well as mine at high speeds.

Nitoragro · Senior Member

I just got a meter from powerwerx but i haven't finished installing it yet. I've never used one so I don't know how the batteries run/drain during a race. It seems with your batteries you are dead or close to dead at 11.25 volts. If I still had about 12v in each battery (i have 3) would i still have another 10-15 minutes? or would the voltage start falling more rapidly after 12v?

mhodgertt · EA President

Under load like I said my batteries start at over 26 volts drop to 24.? soon and then the drop slows but it still slowly goes down. At about 30 minutes of race they tend to be in the ~23.2-23.6 range and by the last lap I like it in the 22.5 range at full power. This is about yhe 11.25 volt range of each battery you are talking about. I have been know to overdrive mine and end at around 21 volts total or 10.5 in each but by then you have really slowed down. Again these are read under power.

I put in the first real meter I have ever used part way through the last spring season so am still learning. But what I did with my meter, I drove it the first time with a set of batteries and watched the amps/volts and kept track of the Watt/hours or the Amp/hours used. Near the end of the race if I slow down just a touch but still moving well, I read that I used a total of 39.2 Amp/hours. Once I know that then I planned on using 3.9 amp/hours every 6 minutes to get the full stored power out. If in the last 5-10 minutes my voltage is still looking good (high 22.? volts) then I can drive hard the last of the race to catch those just ahead of me. At the end of each race I note the amp/hours used and can tell if my batteries are 'breaking in' and getting better or wearing out and going downhill in the amount of power they use.

I know some in this area overcharge and heat their batteries to make them hold a couple more amp/hours but with 18 to 21 cars in our fleet most years we don't do that. I know I could but I don't want to have any advantages over my students so we all just keep them charged in our battery truck at 14.2+ volts each.

Most of our course are full of tight corners and at least 2 of them have a 50 foot+ elevation change so we have to drive very conservative or we die very soon. With that said the slowest winning speed last year was an average of 23.4 miles in the hour. So most of our cars corner very well so we never have to slow on corners very much.

ProEV · Senior Member

Ryan,

The care and feeding of batteries is complicated and key to going fast. There is a great deal of information on the Internet that will give you more background. Here is one example: https://pssurvival.com/PS/Batteries/Lead_Acid/Lead-Acid_Battery_State_Of_Charge_Vs_Voltage_1993.pdf.

In theory, if you read the resting voltage of a battery after it sits 12 hours, you will get an accurate reading For state of charge. This is not very practical. Mike's suggestion of putting a little load on it when you read it, is a good one. This will work best if you make the load exactly the same each time. Look at the graph on the third page of the link above. That gives the voltage curves under different loads. This is the curve for a flooded lead acid which is different from AGM lead acid.

A 12 volt battery is made up of 6 cells. Each cell will read about 2.12 volts when fully charged and 1.75 volts when fully discharged. If each cell has exactly the same capacity, then the battery will have a resting voltage of 12.7 when fully charged and 10.5 volts when fully empty. Cells are never the exact same capacity, so one cell might go to hit empty while the other cells still have good voltage. If the cell is discharged any more, the empty cell will reverse and act as a resistor, using power rather than providing power. That is a 'dead' battery. The car might still run but the most of the power from the good batteries will go to overheating the dead cell.

mhodgertt · EA President

The dead battery issue is why Willamette pushed to get the Yellow Top batteries approved. The Red tops were better in some ways but they seemed to lose their competitive edge too soon. They took 2 to 4 races to break in and then after about 15 more races they had trouble making the full race at the speeds they could the year before. We did not use any battery monitoring meters back then but by the end of the second year I always got 1 to 3 less laps than the year before. In the NW if you go to every race there are about 13 to 17 races each year so I had to buy a whole set of new race batteries for each of our 15 to 22 cars (30 to 42 batteries) which took much of the money I was able to raise with grants, etc. The Yellow tops seem to last us at least 3 years if taken care of and still are competitive. My 'race batteries' last year are from 2012 and were still going strong. I am debating on changing them out this year so that they are broken in for my last 3 years of racing. Now I buy about 10-15 batteries a year to add to the rotation from race to practice batteries and then to sell to kids as batteries for their cars. Last year I did not buy any so need to buy 20 this year...anyone know of a cheap place to buy them?

YES I AM RETIRING FULL TIME THIS YEAR AND PLAN ON COMING BACK HALF TIME FOR THE NEXT TWO YEARS TO TEACH MY ELECTRATHON OR INDUSTRY AND ENGINEERING SYSTEMS CLASSES.

SO WE NEED A NEW PRESIDENT SOON!!! ANY IDEAS???

Lutzedarknut · Member

We have run hundreds of races and we have never noticed a difference when using 2 yellows vs 2 reds vs 1 of each battery. However what we have noticed is how long the batteries stay competitive. The red tops only last a maximum of 2 years while we still use yellow tops that are 3 years old. Also I noticed you mentioned heating the batteries and this makes a HUGE difference. I don't follow Electrathon America as we race in our own sanctioned event in Michigan (necaracing.com), but we used to literally put our batteries in the oven and charge them at 50 amps for 90 minutes. This would give us at least 10% more energy in the batteries. More work in = more work out.

meangreen · teacher / board member

I read someplace that the Red Top Optima batteries are good for approximately 30 charge cycles before they start to fall off while the Yellow Top Optimas are good for 300 cycles. I had the red ones when I was still teaching and mentoring a team; their performance dropped off dramatically by the middle of the second year. When I retired from teaching and started racing my own cars I switched to the yellows. I bought 4 of them in April of 2010 and 4 more in December of 2010 (I had 2 cars). I sold those batteries with the cars in January of this year (2016) and they are still running competitively! I don't know how much longer they will last, but I now know that the yellows are good for at least 6 years if cared for properly.

Yellow Top Optimas sell for $244 + tax each locally - I bought my new ones in February on Amazon for $198 each with free shipping and no core charge...

Zaine Stapleton · Senior Member

It would be a long experiment, but buy a set of yellow tops to do testing on. Literally test them by having a set few parameters for load testing the batteries until they go out for good.
I've started to put the month and year on my batteries to know when I got them along with a tally system too. The tally system is to show how many cycles I can get from my batteries to track how long they last.
The most I've every gotten from a set of yellow tops is 23 cycles. I could have gotten way way more but the problem was I didn't have a charger that would cycle the batteries.
I've now corrected this by getting a separate module for my Iota charger called a IQ4 adapter. Every seven days the charger kicks the voltage from 13.6 up to 14.2 volts or so for a pre-determined time.
This keeps the batteries from going stagnate from sitting in the 13.6 volt range. Plus if I was racing a bit more I could have gotten a more accurate read for how many they last.

Mike did mention that his team can get a set to last two to three years of hard racing before they are retired to practice or automotive usage if chosen. Here in the Northwest before doing the four races this year, that translated to seven single day races, four races at PIR, and two two day summer races. PIR was the tricky one since someone would need two sets of batteries to work with (or have a really fast charging setup to recharge the dead batteries in two hours or less), but if doing one or the other at PIR that comes to a total of 13 to 15 cycles per year. That translates to 39-45 cycles total if going for three years for a set of yellow tops with down time over the fall and winter.

In terms for red tops, Willamette has gotten around 15 cycles. Mike told me after 15, the reds start to drop off really hard. By cycle 17-18 they won't last for more than 20-30 minutes at most.

Zaine