Mixing Battries.

Have any of you mixed battery types, an Optima and a regular lead acid battery? They are completely seperate except when the engine is running then they are connected by a continuos enegized solonoid. It seems the Optima do not charge well or is it faulty? The Optima is the newest of the two about 18 months and resides in the back of the XJ (for now). I wonder if the battery sensor inthe OEM battery tray would be more effective; charge higher if the Optima sat on it instead of the liquid lead acid?

You should not mix an Optima and a regular wet cell battery. Due to their construction, they have different cell voltages.

The charge system is going to charge at a fixed voltage regardless of the battery type or mix. This set-up should work ok I think.(that is connecting together only while charging)

Battery in the rear makes me wonder about the quality of the cable connection. Measure the optima voltage right on the terminals while charging. Also if you are deep cycling the optima make sure it is a deep-cycle type (yellow top IIRC)

gjxj said:

The charge system is going to charge at a fixed voltage regardless of the battery type or mix. This set-up should work ok I think.(that is connecting together only while charging)

Battery in the rear makes me wonder about the quality of the cable connection. Measure the optima voltage right on the terminals while charging. Also if you are deep cycling the optima make sure it is a deep-cycle type (yellow top IIRC)

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The Optima which is in the back is a red top starting battery and it is setup to run the house. I am also not too comfortable with it as a starting battery either. It tends to hold for a while then fall flat fast. I need a good deep cycle battrey.

gjxj said:

The charge system is going to charge at a fixed voltage regardless of the battery type or mix. This set-up should work ok I think.(that is connecting together only while charging)

Battery in the rear makes me wonder about the quality of the cable connection. Measure the optima voltage right on the terminals while charging. Also if you are deep cycling the optima make sure it is a deep-cycle type (yellow top IIRC)

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Wrong. If that were true your voltmeter would not change as the engine speed changed. While the alternator has a voltage regulator to prevent wild open circuit voltages, it current limits and will sit at the cell voltage of the battery. The cell voltage of the battery on an Optima is lower than a wet cell battery. When the Optima hits it terminal voltage (fully charged) it is slightly below the fully charged voltage for the wet cell. The wet cell will charge, but very slowly for the last portion of its capacity.

LOL. Shouting doesn't make you right. If you think the charge system is current regulated please explain how the system distinguishes battery charge current from the current that goes to the rest of the system.

The system is neither a completly voltage or current controlled system. It takes features from both. The key is that the cell voltages are different.

gjxj said:

LOL. Shouting doesn't make you right. If you think the charge system is current regulated please explain how the system distinguishes battery charge current from the current that goes to the rest of the system.

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I agree with old man. This is straight from the FSM:

The amount of DC current produced by the generator
is controlled by the EVR (field control) circuitry
contained within the PCM. This circuitry is connected
in series with the second rotor field terminal
and ground.
A battery temperature sensor, located in the battery
tray housing, is used to sense battery temperature.
This temperature data, along with data from
monitored line voltage, is used by the PCM to vary
the battery charging rate. This is done by cycling the
ground path to control the strength of the rotor magnetic
field. The PCM then compensates and regulates
generator current output accordingly.

Thanks Thayer for the info. I've been an Electrical Engineer for over 30 years, but I didn't feel like taking a page to describe the intricacies of battery charging.

I suspect that gjxj in debating old_man on this subject is unaware of the latter's profession. I happened onto this subject the other day in connection with boating where you are no doubt more likely to find mixes of standard lead-acid batteries and AGM type (Optima) batteries. I understand that the Optima website cautions against attempting to charge the two types in parallel but I have been unable to open the site to see exactly what they say.

If I understand correctly, the AGM battery with it's lower cell voltage should charge first (correct me if I'm wrong). This seems to be contrary to results of techno1154 and I wonder if gjxj is correct in questioning the wiring to the rear location. I'm sure many will know the formula for correct wire gauge offhand but a forum search will provide the chart for gauge based on length and current draw.

I see that old_man replied while I was composing.

You need to look at a battery isolator which allows charging each battery at it own rate.

I want to thank everyone for their reply.

Contrary to the popular concensus, the Optima battery only charge properly when the charging rate/voltage is high. The days when the temperature here in South Florida is above 80* as is most days the charging levels off between 13.5 and 13.8 volts. The couple of times so far that the start of the day was in the low 50's the charging voltage get as high as 14.2 volts. At 14.2 volts the Optima battery maintains its charge more than 2 as long.
The results are the same wether or not the Optima is on the OEM battery tray or in the back of the XJ. This is true when using only the Optima or having the two batteries in parallel. However, the lead acid battery is very happy with a charge as low as 13.5 volts.
Because of this, the Optima is relegated to being only a house battery (although it is a Red Top or starting battery) and the other is entrusted to get me home at the end of the day.
One interesting note is when the Optima is charged with an AC charger, the charger finishes when the voltage reaches 14.4 volts. One would thing that at this voltage, the cells would get hot over night. They do not. Then and only then the hold over time is satisfactory. In the future, I will stay away from Optimas.

Well I didn't know about the temperature compensation on newer Jeeps.
Mine must be broken because I sit at 13.8 all day long.(1996 also) We should expect it to initially be >14 regardless of the battery type or charge state since it will always take some time to heat up.

Techno are you deep cycling the battery? You said its a 'house' battery, but that doesn't mean you are deep cycling it. If you are say running an amp only while driving you dont need a deep cycle battery. On the other hand if you are depleting the battery running stuff for extended time with the engine off then you may have an issue even with a lead acid battery because the charge rate will be fairly low. Even if you put the temp sensor on the house battery you may need quite a lot of driving hours to charge.
(and holding the start battery high isn't ideal for it either)

GJXJ,
I simply atribute it to low voltage regulated by the ECM and or defective battery. I drive more then 100 miles per day with 80% +- on the highway. That should be morre than enough to charge the batteries with a 135 AMP alternator from a Grand Cherokee.
The most interesting thing is, when the charging voltage is beetween 14 and 14.2 volts, the battery holds the charge for days. When the charge voltage is constantly at 13.8 volts or below the battery last only hours.

The only drain I have on the Optima which is rated at 800 AMPS is a 14,000 watt inverter to run my computer and a couple or roof lights when I am out of the XJ and leave the door open.
The battery wires have been upgraded to 4 AWG from the alternator to the PDM and 2 AWG to both batteries for both positive and negative leads. Does the alternator ground also need to be upgraded? I think not. However, if it is necessary, I could make and install some in lees time than it takes to drink a beer. I have plenty of cables, crimp lugs and crimper.

okay i don't know where this is going but. the charging system is designed to charge/ maintain one battery. at a certain amperage. when you put two battery's in you then divide the current to the two battery's providing this is a perfect situation. but variables will exist thus making the battery in the back "optima" charge slower. so each battery will take twice as long to charge.


this is only my look at it.

ps. is the alternator in a 90 xj 100 Amp

90xj06 said:

okay i don't know where this is going but. the charging system is designed to charge/ maintain one battery. at a certain amperage. when you put two battery's in you then divide the current to the two battery's providing this is a perfect situation. but variables will exist thus making the battery in the back "optima" charge slower. so each battery will take twice as long to charge.


this is only my look at it.

ps. is the alternator in a 90 xj 100 Amp

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Thanks for your responce. The question I was trying to ask was, Have anyone mixed batteries and with wath results regarding charging them?

The XJ as well as say 95% of all vehicles were designed with one battery. That does in no way limit the charging system to only one battery. In the marine industry multiple batteries are more of the norm than single battery wetehr they be 6 volts in series ot 12 volts in paralel charged by a single alternator that had a maximun output of about 45 amps and they charged well. Several batteris together increase the amps load on the alternator and therefore the time it takes to fully charge them.

No I have transplanted a 136 AMP alternator from a Grand Cherokee into my XJ 2 years ago replacing the OEM 95 AMP unit when I went to dual electric cooling fans.
Thanks to all who have responded. It appears that no one on this board have installed in their XJ at the same time an Optima battery and a regular old liquid acid battery.
Some one has got to try things first then let others know the outcome. :smsoap: :yap:

It appears that no one on this board have installed in their XJ at the same time an Optima battery and a regular old liquid acid battery.
Some one has got to try things first then let others know the outcome. :smsoap: :yap:

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Because we know better.

Let’s see if I can explain this in two paragraphs. Imagine 2 buckets; one is 12 gallons the other 13 gallons. Both buckets are connected by a pipe. Dumping water into the 12 gallon bucket will net 12 gallons in each bucket no matter how much extra water you add to the 12 gallon bucket. The 13 gallon bucket will not fill up past 12 gallon point unless there is some sort of valve in the pipe. Even with a valve installed you would have to overflow a lot of water in the 12 gallon bucket to get the 13 gallon bucket to fill.

Now, you have two batteries; one reaches full charge at 12v the other at 13v. They are connected in parallel. You will first charge the 12v battery then any extra charge to that battery will be bled off as heat (water running over the edge of the bucket). It will be very hard to get that 13v battery “filled” because the 12v battery is going to be “overflowing” the whole time (and heating up). Once the 12v battery heats up it triggers sensors in the tray to decrease current to the battery, because extra heat is the signal to the ECM that the battery is “full.” A diode between batteries (isolator) will not help the 13v battery charge but will keep whatever charge it does gain over 12v from bleeding back into the 12v battery when the current is shut off (like the valve in the pipe).

Note: Do not switch the 13v battery to the tray because it will not heat up as fast to signal the ECM, yet the 12v battery in the back will be boiling. Solution, get two batteries that are as identical as possible (same voltage, make and model).