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How Often to Change HP-35 Batteries?
The HP-35 has become a coveted device for both collectors and fans. Its legendary status was earned by serving as the first commercially-produced handheld calculator capable of performing transcendental functions. So it goes without saying that special care must be taken to maintain its vintage state of operation, particularly concerning its power pack.
In this blog, we will understand the battery life of the HP-35 and strategies to increase its lifespan.
Understanding the HP-35 Battery System
The rechargeable battery pack that was sold with each HP-35 was a nickel-cadmium (NiCad) type. These batteries were designed for portability and convenience but have limitations due to their chemistry and age:
- Original Battery Configuration: Three size AA NiCad cells (1.2 volts each) combine to give 3.6 total volts in a battery pack.
- Battery Life: NiCad batteries wear out over time, usually after a few years of use. However, vintage HP-35 calculators often come with batteries that are long past their prime.
- System of Charge: This calculator uses a charging jack, so a new battery must be used to prevent leaking or damage if an old battery is charged.
How Frequently Must You Change HP-35 Batteries?
When and how often you need to change batteries will depend on a number of factors:
1. Original vs. Replacement Batteries
If your HP-35 is still connected to its original 1970s NiCad battery pack, then you need to replace the batteries within a year because these cells lose their capacity over the years.
If you’ve upgraded to modern rechargeable NiMH (nickel-metal hydride) or lithium-ion AA cells, they will last up to two years in good condition.
2. Frequency of Usage
For daily users, batteries can last anywhere between 10 to 15 hours of heavy use, after which time they must be recharged or replaced. Casual users with less frequent use may be able to use the battery for weeks or even months.
3. Storage System
Proper battery care (i.e., recharge after power loss) will ensure the best battery lifetime.
Do not store the calculator with dead batteries for extended periods because they may cause leakage. If the calculator has dead batteries for a long time, you’ll require more frequent battery changes.
Signs It’s Time to Replace Your Batteries
Do not let these signs go unnoticed because they will tell you if the batteries of your HP-35 need to be replaced:
- Shortened Life Cycle: If your HP-35 dies quickly after being fully charged, this indicates charged ions are no longer freely migrating between the cells.
- Flashing/Dull Display: LED dimming or flashing indicates low battery.
- Charging Issues: Overheating during charging cycles or failure to hold a charge directly implies battery life.
- Corrosion or Leakage: The battery pack should be immediately replaced if you notice visible evidence of damage to avoid severe internal damage to the calculator.
Best Practices for Battery Maintenance
It goes without saying that you need to maintain your battery for the best results. Here are some tips to help you maintain your battery life:
- Upgrade to modern rechargeable batteries to ensure a long lifespan. You have two options: NiMH batteries and Lithium-ion batteries. NiMH batteries have more capacity but self-discharge quickly compared to NiCad. On the other hand, Lithium-Ion batteries, like those used in a lithium ion solar battery, have a long life and little self-discharge. It just needs a compatible charger.
- Use a timer or a “smart” charger to prevent overcharging because overcharging makes the battery heated and damages the power.
- Own a second battery pack to give you continuous power and save downtime during lengthy recharges.
- Check for battery leakage inside the battery components. If leakage is found, clean the terminals with a cotton swab moistened in isopropyl alcohol immediately.
Choosing the Right Battery Type for Your HP-35
There are different types of batteries that can be used for the HP-35. For example, original NiCad batteries are affordable and compatible to the calculator but drain quickly. On the other hand, modern lithium-ion batteries provide very high capacity, an extended lifespan, and minimal self-discharge. Here’s a comparison table to help you understand their unique characteristics
| Feature | NiCad (Original) | NiMH (Modern) | Lithium-ion (Modern) |
| Capacity | Moderate | Higher than NiCad | Very high |
| Self-Discharge Rate | High (10-20% per month) | Moderate (7-10% per month) | Low (1-2% per month) |
| Lifespan (Recharge Cycles) | 500-1000 cycles | 500-1500 cycles | 1000-3000 cycles |
| Memory Effect | Significant | Minimal | None |
| Compatibility with HP-35 | Fits perfectly | Fits with slight adjustments | Requires proper voltage adapter or charger |
| Cost | Low to Moderate | Moderate | Comparatively High |
| Environmental Impact | Contains toxic cadmium | Safer, but still has disposal concerns | More eco-friendly if recycled properly |
How to Replace HP-35 Batteries
Replacing the HP-35 battery pack is a straightforward process:
- Power Off the Calculator: Make sure your device is powered off to prevent electrical issues.
- Disconnect the Battery Pack: Gently take the old pack out of the battery terminal.
- Examine the Battery Terminals: Clear any corrosion or deposit where the battery is installed.
- Replace the batteries: Replace a compatible battery pack or insert AA batteries into the battery compartment.
- Check the calculator: Turn on the calculator to make sure the new batteries are working correctly.
Extending the Lifespan of Your HP-35
Aside from battery care, here are some tips that may help you keep your HP-35 in good operating condition for many years.
- Keep at a reasonable temperature: Any overheating or humid environment will discharge the battery and risk damage to the calculator system’s internal electronics.
- Replace the charger to a modern one: It is easy to maintain a modern charger rather than having to take care of the original charger every now and then.
- Runoff AC Power: If you can, plug your adapter in for long computing sessions to save your battery for when you need it.
Conclusion
Changing the HP-35 batteries is a small but essential part of maintaining this vintage calculator. How often you replace them depends on the type of batteries you use, your usage habits, and storage practices.
- For modern replacements, expect to replace them every 2-3 years under regular use.
- Always monitor for signs of wear, such as reduced performance or leakage, to prevent damage.
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Common Questions
Can I Use Non-Rechargeable Batteries in the HP-35?
In special cases, the answer is yes. Alkaline batteries that are not rechargeable can give the expected voltage, but these must never be charged in the device. Non-rechargeable batteries are used for the short term only.
What Happens If I Don’t Replace Old Batteries?
The old or leaking batteries need to be removed lest they cause irreversible damage to the circuitry of the calculator and which may even call for expensive repairs.
Are There Alternatives to the Original Battery Pack?
Yes, some hobbyists of HP-35 make their own battery packs incorporating standard AA cells and 3D-printed shells. This type has better performance and is relatively more durable than the conventional type.
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