I recently installed an alternator in my 49 flathead engine (2 actually) which had an 300 cca AGM battery installed. The alternator is a Powermaster Shorty. While the gauge showed 14 vdc, my multimeter showed really wild voltages: -3 to 100 vdc and erratic vac. I discussed it with Powermaster and Speedway. Thinking there was a problem with the alternator, I sent it back under warranty and got a replacement. Same issue. I called Powermaster again, the guy I talked to this time helped me troubleshoot it. I ran the alternator output to my pickup's standard battery and the problem went away. I'm OK with running the pickup battery for now, but really liked the AGM for it's size and mounting flexibility. Besides, the pickup needs it's battery back. I know others are running AGM batteries with no issue, so am looking for advice and maybe a recommendation.
Op check out PowerHouse Performance https://www.powermastermotorsports.com/racing_alternator_features_a.html For Technical requirements and usage . There From what I understand some AMG & Lithium Batteries , the alternators are specific and adjustable to the battery requirement. Some battery specs and information below https://braillebattery.com/
When trying to measure voltages on a car with a "standard" ignition system (copper or graphite leads, points, coil, etc) some digital meters will be all over the place as a lot of interference is generated by the ignition system. Usually the closer to the engine that the meter is operated at, the worse it is. When I strike this PITA, I revert back to an ****ogue meter.
Yep, same experiences. I bought a new Klein multimeter last year and I hate the thing. Dances all over the place. Wish I had a Simpson 260!
Sounds like one of two issues. Either you are seeing the digital meter giving you too much info, which might be useful on a digital system but is too much for your ****og system or, The battery requires a more smooth power output than it's getting from the alt. An Alt. produces AC which is changed to DC using diodes. You did not note any issues so far. You have contacted the Alt maker and seller. All good. If you are still concerned, you can contact the battery maker and try to get confirmation that the chosen parts will work together or just run it and see if they work. I hope you can get a clear answer and share it with us. With tech constantly changing and old stuff being phased out, we are at the mercy of trying to keep our old stuff going with newer replacements.
****og meters like old Simpson 260’s are the most accurate. I think I still have 2 from my old electrician days! They are the only thing than will read correct voltage on my 39 Indian motorcycle that has a 3 brush 6 volt system and a cut out. I use an AGM sealed battery in an old stock case.
I have the large one wire, Powergen alternator and a 925 CCA, Odessey AGM battery in my 34 roadster. It has been on the street since 2019 with zero issues from the charging system. I run the same battery in my 41 PU with a one wire, GM Alternator without issue. I also run the identical battery in my 442, GM, one wire alternator, no issues there either. None of the cars are on trickle chargers. All of the cars have their batteries mounted quite a ways from the alternators, in the trunks or under the seat (PU). All of them start when asked even after long winter naps.
It seems like I read/saw/imagined(?) that ****og meters were not supposed to be used on any computerized vehicles. I know that this isn't a computerized vehicle that @paulsherman is asking about, but wanted to clarify when and when not to use an ****og. Am I remembering that correctly? As for AGM batteries, thats all I use on my vehicles any more. I like the fact that when one does get discharged, it can still be brought back to decent condition. Thats why they use them on mobility scooters.
Thanks - this is an Odyssey battery as well. good to know it's working well. I'm most concerned that the widely varying voltages could damage the HEI.
Both of my DMMs had the same results - Voltage fluctuating with the AGM battery and solid with the lead acid. I double checked all combos with both meters and got the same results. Which narrowed it down to the battery. I have been using DMMs for a while - starting with the Fluke in 1976 (nixie tube display). The auto voltage gauge is steady at 14 with either battery. My biggest concern is if this is feedback/switching voltage, the spikes could damage the HEI. I'm tempted to drag the Tektronix O=scope downstairs to look at the signal.
I don't have an issue right now because I'm using the pickup's battery, not the AGM. Don't want to risk damaging the HEI.
if you are measuring resistance, or output from a low power sensor, then the Ohm/Volt rating of an ****og meter will probably result in incorrect readings. they used to make VTVMs with high Ohm/Volt ratings for this reason. For measuring battery voltage, and ****og meter will work best. But.... this is what you really want to do, to see what's happening. Also there are modern handheld scopes that are not much larger than a typical DMM, that will show you what you want to see.
AGM batteries are not always a wise choice for our older vehicles. One of the major drawbacks for them is that they require a lot more voltage control to not over charge. No, the 14.5 volts that your alternator puts out won't blow it up, but it will dramatically shorten its useful life. This is why most manufacturers suggest replacing the battery with a like type battery as AGM equipped vehicles usually (I say usually only becuase I can't confirm they all do) have the proper charging controls. It is also why they recommend using a battery charger with an AGM selectable option to bench charge them. If anyone knows of a charging system regulator for AGM batteries in our older hot rods, please post it up.
It's got to do with input impedance (ohms/volt)- Some old ****ogue meters would "load down" the circuit that was being measured, so incorrect values could be read. The higher ohms/volt rating, the less load on the circuit. Digital meters have a much higher rating, so the lower ****ogue signals in a modern car can be read accurately. An ****ogue meter may have a rating of 20K ohm/volt, whereas a digital meter will be up in the 1million (1megohm)/volt range.
If you really want to know if it's ripple from the alternator or spikes from the ignition you probably have to do just that. I'm guessing one battery just happens to be better at dampening the voltage changes that confuses your digital multimeters - the battery doesn't cause any problem, it just doesn't hide problems elsewhere equally well as another battery. Then again, there IS a possibility of a bad battery loading down the alternator causing it to work harder, also producing more ripple at higher load.
You mentioned HEI as a concern twice. Are you talking good old GM High Energy Ignition (HEI)? If so, they are pretty reliable, easy to service and pretty cheap for decent parts - just don't buy offshore junk. The big thing is they are darn near impervious to all sorts of ignorance, neglect and even high or low voltages. I've seen them work with the old resistor in the supply along with funky wiring, plenty of cars with bad charging systems (high and low or no voltage) that didn't need ignition work after repair. They seem to mostly just work. They will burn through the cap and rotor at times, but that's usually after the plugs have worn an additional 20+ thousandths, the fueling has gone lean and the wires are shot. The module can go, but it's 2 connectors and a couple screws.
AGM type batteries aren't able to shunt all transient energy as well as a typical lead acid type, due to their low internal resistance.
More likely you talked to Ed @ powerhouse , If not Talk with Ed . You mentioned batt out of Truck issue problem went away ? Correct ! Is the Truck Batt & the 300 Amg same Group Size ? I am thinking truck Batt a bigger Group size & cca More of a Reserve Amps per Hour. About 15 years ago I used a battery Called Batt Battery it was around 300 cca Amg No bigger then a H-D sportster battery, it would start my 408 sbc with 11:5:1 OEM Mini Gm starter ,, spring to Fall , WINTER No 2-5 sec like dead Battery, I switched to the Amg Oltima 34/78 group , battery under car sedan no access mounted to the frame. By chance do you have Access to a different size AMG to test ? All these different AMG manufacturers have different specs on the same group size of batteries , I have not tried Odyssey , But my Race car has a Optima For 20 years , So I stick most time brand & there was a No h***le exchange for five years , I probably have 10 different Optima's .... For the HEI In since 1975 also fleet mechanic technician I have only mabe had hand full of the HEI modules go bad , As of Now I have 4 GM HEI's vehicles with AMG batteries No issues But I can Say all have GM OEM modules no aftermarket. Between Ed & Jim squirrel here on HAMB I narrowed my issue down. I have a few Questions on abbreviations used the meaning, Erratic VAC = Voltage @ Cranking ? & -3 to 100 vdc = Volts Direct Currant ? If so -3 to 100 ? Minus 3 Volts ? My self I have learned more about battery group Sizes , Reserve , Draw Amps @ Per Hour, Alternator speed , Some Alternators only put out X amount of amps below 2,500 RPM's @ alternator speed , Then Jump to max amp out put above the 2,500 rpms of Alternator speed ,,, Some have a 3 speed Amps out put , idle below 2,500 Alt Rpms , Cruise Amp 2,500 - 8,000 Alt Rpms , then Max Amp out put @ 8,000 Rpms of Alt speed ... Denso has max of 20,000 Alt Rpm's Gm has max of 16,000 to 18,000 alt rpms.. Then charger Wire Awg size Requirements for how many Amps Alt puts out , & distance between Alt & Batt. & Cable sizes ForBatt Group , Starter draw & how far ( length of wire) from Batt to starter . I know I'm hard to understand but maybe some of the information I tried to explain can help ,,
Responding: Erratic VAC = Voltage @ Cranking ? - Volts Alternating Current. Of course, it should be zero... -3 to 100 vdc = Volts Direct Currant ? - yes If so -3 to 100 ? Minus 3 Volts ? - yes, minus 3 vdc. The pickup battery is 700 cca I have an Optima in my 40 Chevy, I don't recall if it's AGM, it should be. I'll look at the Optima 34/78 group as well. I was hoping to mount the battery sideways under the back seat. However, the body is off the car and it wouldn't be that hard to make a well in the floor under the seat to hold a standard battery. I used PowerMaster's 8 ga wire sets. 12 inches for the ground and 3 feet for the positive, as battery sits on a platform behind the starter. The alternator is 48 amps at idle and 90 at 2000 rpm (if memory serves, the info is in the garage).
@paulsherman Yes you can mount the Optima on side I think even upside down , One mine is on side & flat . My issue was only when E fan on for more then 15 mint continuously @ Idle. I switched to Yellow 34 different then Red 34 ,, spec's . I also needed got to Awg #4 charge wire. My Denso was 105 amp , but @ Idle under 2,500 rpms "Alt" pulley speed @ Idle was 35 amp out put , When E fan turned On with other Electronics & lights , all pulled 47 amps @ idle , So I needed to go to the 150 amp Denso @ idle 55 amp out put . Tight space Alt location sbc where fuel pump would be PS frame rail . Also the Denso Max 20,000 rpm of Alt shaft speed ,,,, Gm max 16,000 to 18,000 rpms. I looked up the ""Odyssey"" the only battery I could find was ""roughly 310 CCA"" ,,, Just for reference & comparison between manufacturers just study the photos Red top one 800ish cca , 34/78 Yellow Top 34/78 difference between the two YellowTop then the Yellow top with 450 CCA Then the Odyssey Then another lithium battery I have.. Just look over the Specs ""Odyssey "" & Look @ The Lithium specs Close to 2 hrs @ 90 amp reserve
Mine is the ODS AGM28LA Weirdly, it doesn't come up in their matrix, but a version shows up in the search - https://www.odysseybattery.com/products/ods-agm28l-battery-pc925/
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